Lesson 56 - Introduction to IPv6 - Address Structure

After a very short introduction to IPv6 types of addresses and the header presentation, let's dig deeper and find out more about IPv6 addresses.

Hexadecimal Address Notation
I have already mentioned that IPv6 global space begins with 2000::/3. This means that all public IPv6 addresses now use three bits which amount to value 2 in hex.

Look at this notation of global address one more time:

2000::/3

This is a shortcut for:

2001:0000:0000:0000:0000:0000:0000:0000/3

And just like in IPv4 addresses this '/3' tells us to pay attention to the three most significant bits. Hexadecimal to binary conversion is simple. Two digits in hex (for instance: 20) is a one byte value (eight bits). One character stands for four bits (called a nibble). They need to be converted as two separate entities.

Pic. 1 - Hex-to-Binary Conversion.

This way, the hexadecimal 20 becomes 00100000.

In the global IPv6 address scope the first three bits ('/3') will always be set like shown below (as of writing this post):

Pic. 2 - IPv6 Global Address Sequence of Bits.

Okay. What about the hierarchy in IPv6 address?

IPv6 Hierarchy

It is not very different from what we used in IPv4 classes! Do you remember these bits settings in IPv4? If not, look at the below numbers:

Class A - 0xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx /8
Class B - 10xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx /16
Class C - 110xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx /24

The blue color denotes the network portion of the address in these classes. For instance, the '/24' means that 24 most significant bits are the network bits. The remaining ones (eight of them left) are the host bits. This way we create the hierarchy in IPv4 addresses. 

What does it look like in IPv6 then? It is much simpler. See for yourself:

2001:acbd:1234:a1b2:0000:0000:0000:0001/64

The first sixty four bits (blue) are network bits, the remaining ones are the host's interface identifier (host bits). But this, we already know based on the previous post. The network portion is going to be given to us either by the ISP or by the registry (ARIN in the US, RIPE in Europe etc.) if we're a big organization. 

We're told that the length of the prefix assigned to different organizations looks like this:

• Registries will be assigned /23
• ISP will be assigned /32
• Sites will be assigned /48

Say, you are the organization that receives /48 prefix like this:
2001:1234:5678:0000:0000:0000:0000:0000/48.

This gives you two bytes (shown in red) in the network portion to create different networks (red portion: 2^16=65536 different numbers). By the way, get used to the shortcuts:

2001:1234:5678:0000:0000:0000:0000:0000/48 = 2001:1234:5678:0000::/48

IPv6 Interface Identifier (host bits)
How do we get the host portion then? 

In terms of creating the host portion of IPv6 address, if DHCP is not used, we have two options. One way is to do this by hand like presented below (the red color is the interface identifier or the host bits if you will):

2001:1234:5678:1000::1
(abbreviated address which could be expanded to 2001:1234:5678:1000:0000:0000:0000:0001)

But this approach seems somewhat tedious. I mean, who wants to assign addresses on 500 hosts this way?

Another way is to use EUI-64 method which creates the host portion of IPv6 address based on the MAC address of the first Ethernet interface which, at least in theory, should be unique!

Because the MAC address has only 48 bits and not 64, a special procedure has been developed to fill in the gap. Check it out the MAC address of the interface on the router:

Pic. 3 - MAC address of F1/0 Interface.

In the EUI-64 method the operating system takes the MAC address (highlighted) and uses it to create the host bits for the link-local address by default (you can also do this manually). And the same method can be used to assign the host bits in the global address.

Here is the detailed description of how 48 bits of MAC address becomes 64 bit host portion of IPv6 address in EUI-64 method (operating system takes care of these details itself).

Step 1
Flip the 7th bit (counting from left to right) in the MAC address (if it's 1 change it to 0, if it's 0, change it to 1).

In order to do that, operating system must first convert the hexadecimal number of the first byte to the binary form. MAC address used in the example is (as per pic. 3):

CC00.19BA.0010.

(Hex) CC ==> (Bin) 11001100

C=1100

Counting from left to right the 7th bit is: 11001100. This one must be flipped like shown below:
11001110. This gives us the hexadecimal value of: CE.

Step 2
Use the MAC address after modifying the 7th bit in step 1 and inject the FFFE between OUI (first three bytes on the left) and the serial number of the adapter (first three bytes on the right). I will use the colon (:) as the delimiter and blocks of 2-bytes to make it compliant with the IPv6 convention. Look at the result below:

CE00:19FF:FEBA:0010

This way we get 64-bit host portion of the IPv6 address. So my link-local address which uses this method to create the host bits will look like this:

CE00:19FF:FEBA:0010, which is abbreviated to this form:
CE00:19FF:FEBA:10

Step 3
Add the network portion to this construct calculated in step 2. For instance, we know that link-local's network bits are always FF80::/10. So, the whole link-local address is going to be (abbreviated as per Lesson 55's explanation):

FF80::CE00:19FF:FEBA:10
(again, the full 128-bit version is: FF80:0000:0000:0000:E00:19FF:FEBA:0010)

Let's see if this is true when I enable IPv6 support on F1/0 interface. Pay attention to link local address value.

Pic. 4 - IPv6-enabled F1/0 Interface.

Note that just by enabling IPv6 support on the interface IOS automatically creates link-local address using EUI-64, allowing this interface to communicate with hosts residing on this local subnet/network. However, in order to communicate with hosts outside this link-local network, the interface needs a global address as well. This leads us to a correct conclusion that each interface will have two addresses: link-local and global in order to fully operate in IPv6 domain.

IPv6 Global Address Configuration

There are two major methods of configuring IPv6 addresses on our devices:

• Manual method - You will configure them like we sometimes do with IPv4 right now.
• Dynamic method - This one offers us IPv6 DHCP servers or the hosts can obtain the network portion of the IPv6 address from their local router (special message is sent called 'Router Solicitation'). In the latter situation the host portion will be generated automatically using the EUI-64 method described above.

Since this post has enough information for one sitting, I'll show you the manual and router solicited methods of configuring IPv6 addresses in my next post. I will also explain what happened to our IPv4 ARP request since broadcast does not exist in IPv6 world anymore!

Lesson 54 - Network Address Translation Part 4

It's time to put our theory into practice. We're going to use a single topology and try out different scenarios. They are not related to one another but my intention is to show you the variety of different methods in use. Keep in mind that they are just the few fundamental types of translations but there are more options available and they can become a bit more complex than the ones presented here.

Pic. 1 - Network Topology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

R2 is playing a role of ISP's router. For the purpose of this presentation I'm not going to use real IP public addresses. I'm going to use 172.x.x.x range and pretend they are public ones.

Addresses used in this lesson are going to be as follows:
Private (LAN) IP = 192.168.1.0/24 and 192.168.2.0/24
Public (Internet) IP = 172.16.1.0/30 (connection to ISP)
Public (Internet) IP = 172.30.1.1 (Internet host used to check NAT)

All these addresses are simulated using Loopback interfaces.

R1 Initial Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
!

ip http server

!

ip route 0.0.0.0 0.0.0.0 172.16.1.2

!

R2 Initial Configuration:

!

interface Loopback0
 ip address 172.30.1.1 255.255.255.0
!

interface FastEthernet1/0
 ip address 172.16.1.2 255.255.255.252

!

ip http server

!

ip route 172.20.1.0 255.255.255.252 172.16.1.1

!

Scenario 1 - Static NAT
In this scenario I'm going to configure a static (one-to-one) NAT. This type of configuration allows me to present my public services to the Internet clients. Let's pretend that our 192.168.1.1 address is a Web server and is connected to R1 (loopback1 simulates this server). Our ISP gave us two public IP addresses (we pretend they are public anyway).

Inside Global Address Pool:

• 172.20.1.1/30
• 172.20.1.2/30

Our web server (192.168.1.1) will be seen on the Internet as 172.20.1.1. Below is the configuration to accomplish that.

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside

!

ip nat inside source static 192.168.1.1 172.20.1.1

!

Note!
When you assign inside or outside role on the interface, the router is going to add 'ip virtual-reassembly' (dependant on the IOS version you use). At the CCNA level you can ignore the meaning of this command.

Verification:

Pic. 2 - NAT Table.

Note!
Static entry is created allowing transmission towards 192.168.1.1 which is seen as 172.20.1.1 on the Internet. Since we have not specified any layer 4 protocols (tcp/udp) and their ports, all ports are open in R1 allowing access to all ports on the web server. If you wanted to open only TCP port 80, you could use this command instead (more likely in real life scenarios):

!

ip nat inside source static tcp 192.168.1.1 80 172.20.1.1 80

!

Pic. 3 - Accessing Web Service.

Connection succeeds!

Scenario 2 - Dynamic NAT using IP Address Pool.
In this scenario we have the same pool of pretend-to-be-public IP addresses from scenario 1. This scenario will dynamically pick the first available IP address and use it for the host that wishes to send packets to the Internet. Since we only have two public IP addresses, only two hosts can send traffic at the time. Adding the 'overload' keyword will include the port translation and more hosts can reuse these two public addresses (172.20.1.1-2/30).

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside
!

!

! The below command defines public IP addresses in the pool.

!

ip nat pool ISP_POOL 172.20.1.1 172.20.1.2 prefix-length 30
!

!

! The ACL1 matches on both subnets, the candidates for translation

! IP addresses that match the ACL1's statements, will be NATed.

!

access-list 1 permit 192.168.1.0 0.0.0.255
access-list 1 permit 192.168.2.0 0.0.0.255
!

! The translation of INSIDE Local IP (ACL1) 

! to the INSIDE Global IP (pool ISP_POOL)

!

ip nat inside source list 1 pool ISP_POOL

!

Verification:

Pic. 4 - NAT from 192.168.1.0/24 and 192.168.2.0/24 Subnets.

This type of translation is not used as often as the last one. In case you wanted to use it and have more than two hosts sending traffic towards the Internet, you would use the same configuration including the 'overload' keyword like presented in the scenario 3.

Scenario 3 - Dynamic NAT using IP Address Pool with Overload.
The same method like presented in scenario 2 but used when there is no sufficient public (Inside Global) addresses for the number of hosts used in our LAN (Inside Local addresses).

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside
!

!

! The below command defines public IP addresses in the pool.

!

ip nat pool ISP_POOL 172.20.1.1 172.20.1.2 prefix-length 30
!

!

! The ACL1 matches on both subnets, the candidates for translation

! IP addresses that match the ACL1's statements, will be NATed. 

!

access-list 1 permit 192.168.1.0 0.0.0.255
access-list 1 permit 192.168.2.0 0.0.0.255
!

! The translation of INSIDE Local IP (ACL1) 

! to the INSIDE Global IP (pool ISP_POOL)

!

ip nat inside source list 1 pool ISP_POOL overload

!

Scenario 4 - NAT Overload
This is by far the most often used translation. This method is used on all broadband connections. In this method we only need a single public IP address (the one we use on the router's interface facing the ISP).

Our Inside Local addresses are: 192.168.1.0/24 and 192.168.2.0/24, and the single Inside Global address is: 172.16.1.1. Check the configuration below:

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside

!

! The ACL1 matches on both subnets, the candidates for translation

! IP addresses that match the ACL1's statements, will be NATed. 

!

access-list 1 permit 192.168.1.0 0.0.0.255
access-list 1 permit 192.168.2.0 0.0.0.255
!

ip nat inside source list 1 interface FastEthernet1/0 overload

!

Verification:

Pic. 5 - NAT Overload.

In case things do not work, use the following steps to troubleshoot NAT:

1. Check the 'ip nat inside' and 'ip nat inside' statements on the interfaces.
2. Check if your ACL matches on the appropriate Inside Local addresses. If you send traffic from these, 'show access-list' should show you the hits against the ACL entries.
3. You could consider using extremeley dangerous command: 'debug ip nat'. This one however, will inevitably crash your production router. If the traffic is not heavy (no users using the Internet), you can try to use it the way I present below.

I will use the debug ip nat for the presentation purposes only. DO NOT USE THIS on the production equipment!

I will trace a specific Inside Local address (192.168.1.1) translation.

Step 1
Configure the ACL that matches on our single Inside Local address (192.168.1.1)

R1 Configuration: 

!

access-list 99 permit host 192.168.1.1

!

Step 2
Using ACL 99 configured in step 1, I will use debug to see the NAT translation work. The proper output is shown below:

Pic. 6 - NAT Troubleshooting.

Note!
Translation: 192.168.1.1->172.16.1.1 when the packet is sent out F1/0. The translation on the packet returning: 172.16.1.1->192.168.1.1.


Also, the ACL receives the hits. Loot at this below:

Pic. 7 - ACL Hits.

Note!
My ACL 1 which matches on Inside Local addresses has received a hit. It is configured correctly for NAT.

Lesson 30 - IPv4 Subnetting - Practice

In the previous post, I showed you three major rules used in calculating subnets. This knowledge can only be verified in practice though. Let me show you a few examples related to subnet calculations. I hope that looking at this topic from different angles is going to help you understand the concept better and feel confident when planning your IP addressing scheme. The first four questions are merely appetizers for a bigger dish: VLSM.

I am going to refer to my previous post's rules while answering the questions (rule 1, rule 2 and rule 3).

If you still do not remember the weights of all bits, you may consider using this little aid presented below (pic. 1) while calculating subnets, and converting binary network masks into decimal values.

Pic. 1- Subnet Calculation Aid.

This tool is useful before you remember all the weights from left to right and right to left.

Pic. 2 - Example of Subnet Binary-to-Decimal Conversion.

Question 1

Given the prefix 192.168.1.0/24, what should be the length of subnet mask allowing up to 9 subnets?

Answer 1

The address belongs to the class C and uses its default network mask. That leaves us with 8 bits to play with (the last byte). Before we change anything, our address and network mask converted into the binary notation look like shown below (pic. 3).

Pic. 3 - 192.168.1.0/24 in Binary.

In order to create 9 subnets we must extend the existing length of the network mask by 4 bits which allows up to 16 subnets (use calculation aid in pic. 1). If I tried to extend it by 3 bits only, the maximum subnets allowed would be only 8 subnets (rule 2 in lesson 29). So, I must use 4 bits and the result is: 192.168.1.0/28 (192.168.1.0 255.255.255.240).

Pic. 4 - The Answer to Question 1

Question 2

Given the host address 192.168.1.177/29, what are the subnet and broadcast addresses?

Answer 2

In order to determine the subnet and broadcast address of the subnet of this host address, we must look at the length of the network mask first. It is 29 bits (24+5). This tells us that the last byte of the address has 5 bits masked (subnet bits) and 3 bits unmasked (host bits). It is a good idea to look at the the last byte of the address (177) with its network mask using binary notation. Pic. 5 below shows you this clearly.

Pic. 5 - 192.168.1.177/29 in Binary.

Since we must determine the the subnet in which the host resides (177 = 10110001), the host portion of the prefix (host bits reside in the last byte) must all be set to '0'. The byte value with the host zeroed is the address of the subnet (rule 1 pkt.1 in lesson 29). This is the result:

Pic. 6 - Host Bits Zeroed = Subnet Address.

The second part of the question relates to the broadcast address of the subnet. As you remember, in order to obtain the broadcast address, you must put '1' on all host bits of the subnet/network. The subnet has already been determined (pic. 6), so let's put '1' on all bits of the host portion:

.10110111
.10110000 = 176 <- subnet address
..00000111 = 7 <- host bits set to '1'

In decimal it is: 176 + 7 = 183
The broadcast address is: 183.

The below picture illustrates it using binary numbers.

Pic. 7 - Host Bits Set to '1' = Broadcast Address.

Question 3

Given the  prefix 172.16.0.0/17, how many subnets can you create?

Answer3

This is a bit tricky isn't it? In order to answer this question, you don't need any calculator, paper or pen. You must trust the rule 2 in lesson 29. The address and its network mask (called prefix) converted into binary look like presented below:

Pic. 8 - The Number of Subnets for 172.16.0.0/17

As you see the number of bits we have extended the class B address is: 1. So, the number of subnets we can create with it is: 2 subnets, since this subnet bit can be either 1 or 0.

Pic. 9 - Questions 3 Answer

Question 4

What length of network mask would be the most optimal for router's point-to-point connection?

Answer 4

The key to this question is to understand that point-to-point connection needs only 2 host addresses (two points that are connected together). Knowing this, the rest is a piece of cake. We use rule 3 inlesson 29 to determine the length of the network mask that allows 2 host addresses. Check out the picture 10.

Pic. 10 - Calculating Point-to-Point Connection Host Addresses.

If you count ones above the optimal network mask for point-to-point connection is /30. The decimal value is: 255.255.255.252.

Question 5 - Variable Length Subnet Masking (VLSM)

It's time for a big one. Given the topology (pic. 11), calculate IP addresses for each subnet trying to optimize them according the host address requirements. The IP address you should use to create subnets is: 192.168.1.0/24. The number of host addresses in the subnets are as follows:

Subnet 1 = 46 host addresses

Subnet 2 = 16 host addresses

Subnet 3 = 10 host addresses

Subnet 4 = 2 host addresses

Subnet 5 = 2 host addresses

Pic. 11 - VLSM Topology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

As always, if you know the rules and the method, it is going to be easy thing to do. The rules have been discussed in lesson 29, so let me go about this kind of task now.

---

NOTICE!
If your design looks similar to mine (optimizing addresses to the number of host required) you muststart the calculation with the largest number of host addresses requirement and work your way down to the least number of host addresses.

---

This is one of the many methods available. It helps quickly calculate all subnet ranges without using calculator (pen and a piece of paper should do).

Step 1
Determine the length of the network mask for each subnet in question. Keep in mind we focus in on the last byte of IP address 192.168.1.0 (8 bits).
The first three bytes do not change!

Subnet 1 = 46 Host Addresses

In order to allocate 46 addresses we must use 6 host bits. Why? 5 bits will not be enough as 2 raised to the power of 5 is 32. Also, we must decrement two addresses for subnet and broadcast addresses. So using 5 bits would give you only 30 host addresses. Here we go with 6 bits then:

Pic. 12 - Subnet 1 in Binary.

Subnet 2 = 16 Host Addresses

We must repeat the same math for the remaining subnets.  How many host bits to allocate for 16 hosts (subnet 2)? We must use 5 bits. In case we wanted to use only 4 host bits, the maximum number of hosts is 14 (16 - 2).

Pic. 13 - Subnet 2 in Binary

Subnet 3 = 10 Host Addresses

We continue using the same logic.

Pic. 14 - Subnet 3 in Binary.

 Subnet 4 and 5 = 2 Host Addresses Each

On point-to-point links only 2 host addresses area needed. The most optimal network mask is /30(30 bits).

Pic. 15 - Subnet 4 and 5 in Binary.

Step 2

Now, that we know the length of network mask for each subnet, we can start calculating the IP address ranges. 

The subnet 1 address is: 192.168.1.0/26.

The value of the lowest bit in the network mask is going to be our increment used to calculate the next available subnet address. With /26 the increment value is 64 (pic. 16).

So, if we add the increment to the last byte, we get the number of our next available subnet address:

192.168.1.0 + 64 = 192.168.1.64.

From there, this next subnet address (value) - 1 is the broadcast of our current subnet:

192.168.1.64 - 1 = 192.168.1.63 (current broadcast address)

Current subnet value + 1 = the first host address:

192.168.1.0 + 1 = 192.168.1.1 (first host address of current subnet)

Current broadcast address - 1 = the last host's address:

192.168.1.63 - 1 = 192.168.1.62 (last host address of current subnet).

Look at the below pictures which illustrate this method.

Pic. 16 - Subnet 1 - IP addresses

Pic. 17 - Subnet 2 - IP addresses

Pic. 18 - Subnet 3 - IP addresses

 Pic. 19 - Subnet 4 - IP addresses

Pic. 20 - Subnet 5 - IP addresses

Now, we're ready to start talking about routing. In my next post, I will talk about a router, its functions,and  basic operation. From there, we'll start exploring routing protocols.

Lesson 29 - IPv4 Subnetting - The Rules

Now, that we have already learned a few things such as conversions between binary and decimal, how to recognize classes of IP addresses based on the 'first octet rule', and what is the purpose of the network mask, we can tackle IP subnetting.
A natural (default) network mask is used with class C of IP addresses quite often. But it is very uncommon to use class A and class B IP addresses with their natural netmask. They are often sub-netted (broken down into multiple smaller networks). This is accomplished by increasing the length of the default (natural) network mask.

---

 Incidently, the network IP addresses that use their natural (default) network mask are called                   Classful Networks.

---

But why do we create subnets to begin with?

There are many reasons why we decide to use subnets rather than classful networks. But the most important is that we want to use IP addresses efficiently since they are a scarce resource these days.

Imagine that you have a huge network to support. It uses class B network address: 172.16.0.0/16. Since the number of bits in the host portion of this address is 16 (the last two bytes are not masked), we can place 65534 hosts in a single network. Even if you used 2000 hosts still it is too much to keep them in one broadcast domain. Can you imagine that many computers sending and receiving broadcasts such as ARP requests? Well, I can imagine that, but it does not mean its efficient. In fact, broadcast traffic would pretty much kill this network. Even with thousand computers that would be way too much broadcast traffic to receive.

If we divide this huge network into multiple subnets with fewer hosts per subnet, we improve the efficiency of the system. A router will connect those subnets to allow unicast communication, but broadcasts will not be propagated between subnets as routers do not forward them. For instance: 172.16.1.0/24 subnet allows only 254 hosts in it. The broadcast will be propagated between this number of hosts rather than among one or two thousands of hosts.

Another reason for using subnets is about relates to public IP addresses that are leased to customers. ISPs do not easily give out whole classes of IP addresses (classful) to companies but rather portions of these (subnets).

Other reasons may be related to security of your hosts. Network divided into chunks with routers as gateways, give you more control as to who can 'talk' to whom.

I use terms such as broadcast or unicast. If you are not sure what these terms mean, let me present brief definitions.

Transmissions:

• Unicast - a single source host sending to a single destination host.
  Example: Src=192.168.1.1, Dst=192.168.1.2
• Broadcast - a single source host sending to all hosts in the network/subnet. Example: Src=192.168.1.1, Dst=192.168.1.255 (more on this address later in the post)
• Multicast - a single host sending to a single group of hosts (IP class D)
  Example: Src=192.168.1.1, Dst=224.10.10.10.

There are three things I would like you to remember before we delve into subnetting.

Rule 1

1. If the host bits in a given IP address are all set to '0', this is the network or subnet address.
2. If the host bits in a given IP address are all set to '1', this is the broadcast address (all hosts in the subnet/network are destination).

Rule 2

The formula used to calculate the number of available subnets given the specific length of network mask.

Pic. 1 - Number of Subnet Calculation  - Formula.

Rule 3

The formula used to calculate the number of available hosts per subnet or network given the specific network mask.

Pic. 2 - Number of Hosts Per Network/ Subnet - Formula.

Before we start using the above rules, let me show you a few examples of network, subnet and broadcast addresses based on what we have discussed in the last three posts including this one. If you do not remember the 'first octet rule', which determines the class and the default network mask of an IP address, use the following table as the reference. The number ranges of the first byte determine the classes as shown in pic. 3.

Pic. 3 - Classful Address Table.

Pic. 4 - Network (classful) Addresses and Subnet Addresses (classless).

In order to determine the number of subnet bits to use them as the exponent in the above formula (pic. 1), you must first know what is the default network mask of the IP address according to its class (pic. 3). Then, you must count the bits that were added to this default network mask. These bits allow a number of subnets to be created as per formula in pic. 1. Check out the below example.

Pic. 5 - Number of Subnet Bits (Example).

In the example (pic. 5), IP address belongs to class C since the first byte value is 192 (compare it with pic. 3). Class C uses first three bytes (24 bits) to denote the network portion of the address. Today we can say that its default network mask has the length of 24 bits (255.255.255.0). Since our network mask length is /28, we have extended the default network mask by 4 bits (bits in the green color). Thus, we get 4 subnet bits that must be used in our formula presented in pic. 1.

Pic. 6 - Number of Subnets Available - Calculation.

Using the same example: 192.168.1.0/28, how many host addresses per subnet can we use?

Pic. 5 shows us that with /28 we have 4 bits left for host (total number of bits = 32). In order to calculate the available number of host addresses we must resort to formula presented in pic. 2.

Pic. 7 - Number of Hosts Available - Calculation.

Make sure you understand how the three rules presented here work. In my next post, I'm going to show you how to use them to calculate the subnets based on different criteria such as:

• Number of subnets per IP address
• Number of hosts required in the subnet
• Number of desired host per subnet - Variable Length Subnet Masking (VLSM)

Lesson 28 - IPv4 Address Dissected - Part 2

In my previous post I have talked about the hierarchy in IPv4 address. It is the network mask that tell us which bits form a network address and which ones denote the host in the network. But there is more to it ...

You already know that IP address and its network mask create a unique layer 3 identifier of a host. This allows devices to communicate. When a computer sends a packet, it will put the address of the recipient in the IP header as the destination. It also puts its own address (sender), so the recipient knows who sent the packet and to whom a reply should be sent.

After reading my previous post it may seem clear what a network and a host in the network are. But you may still ask: "What is a network anyway?". As strange as it sounds, the answer to the question may not be as simple as one might think.

We tend to divide our networks into multiple pieces that are uniquely identified with the network portion of the address (masked by network mask bits set to 1). Individual hosts placed in those networks will also have unique host portion of the IP address (network mask bits set to 0). They are unique in their network.

Why do we break down the company's infrastructure into multiple chunks called networks?

There are many reasons we do it.  Larger networks are harder to maintain and become very inefficient as the hosts receive too many broadcast transmissions (broadcast=one packet to everyone). Also, it is easier to control traffic and tighten the security when dealing with multiple networks rather than doing so in one single organism. So, we separate hosts using layer 3 devices called routers and address them uniquely, thus creating multiple interconnected networks. The routers that connect networks become the gateways between them allowing unicast communication (one-to-one). At the same time they do not allow broadcast traffic (one-to-everyone) to go through. They also are equipped with many mechanisms to control the traffic traversing through them.

How we divide our system into multiple networks may be depended on numerous factors. For instance, a network in the building can be divided based on the floors. First floor is the first network, second floor is the second network etc. Other division could be made based on the departments in the corporation. HR is going to be one network, Legal Dept. another etc. Either way, computers will belong to one of these networks, but all of them together will still belong to the same company.

So, what is a network? A network is a group of devices that share the same network portion of the address. That portion is dictated by the length of the network mask. These bits mask the corresponding bits in the address. But this you already know. However, there are a few more things you need to know.

Consider this picture.

Pic. 1 - IP Networks.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

What do we see in the picture?

In the middle sits a layer 3 device called a router (R1). I will devote a whole post on what it is and what its functions are. Meanwhile, let's carry on with our description.

The router (R1) connects two different networks. It becomes the gateway between them. This means it allows communication between them (unicast by default). The two networks in the picture have the following addresses:

1. The network on the left-hand side has the address of 192.168.1.0/24.
2. The network on the right-hand side has the address of 192.168.2.0/24.

The switches (SW1 and SW2) are transparent in terms of layer 3 communication. Remember? They are layer 2 devices, and do not have a clue what IP addresses are.

Finally we have two computers. PC1 belongs to the network 192.168.1.0/24, PC2 belongs to the network 192.168.2.0/24. Both have the same host identifier in the last byte of their IP address: 1. But their network portion of IP address is different. As a result of that the belong to two different networks. Just like two Mr. Smiths who live in two different houses on two different streets of the same town.

PC1: 192.168.1.1/24
PC2: 192.168.2.1/24

Have you noticed that the third octed (from the left) has different number? That makes the network portions of these two addresses unique.

What does this /24 mean in the address?

As you already know the netmask separates a network from a host portion of the address. Both IP addresses and their network mask are presented in the decimal notation with the length of network mask (/24). Yes, the shortcut /24 means, that the network mask is twenty four bits in length. This in turn, means that the first twenty four bits of the IP address is the network portion. The remaing eight bits are the host address in the network. Check it out below.

Pic. 2 - PC1 address with the netmask 24 bits (/24).

Looking at the binary, we see that /24 = 255.255.255.0.
If we convert 255.255.255.0 into binary we get twenty four 1s in the network mask, giving us three bytes of the network address. The remaining eight bits in the network mask are all 0s. This way, the last byte (8 bits) becomes the host identifier int the network 192.168.1.0.

Interestingly enough, many moons ago, there were no networks mask in use. So, how on Earth, did the devices know which bits of IP address were network bits and which were the host bits? The designers of IP addresses divided IP addresses into five different classes. The first byte of any IP address (on the left) determined which class an IP belonged to and how many bits of the address were network bits. Today, this is known as the first octet rule.

IP Address Classes
The value of the first byte in the IP address determines its class (the first byte on the left-hand side).

• Class A - values 0 through 127
• Class B - values 128 through 191
• Class C - values 192 through 223
• Class D - (multicast) - values 224 through 239
• Class E - (experimental) - values 240 through 255

This stems from the fact that some assumptions have been made regarding the most significant bits in the first octet (byte). The below table shows (in red) these bit reservations. They are fixed in the class and cannot be changed.

Pic. 3 - IP Address Classes.

As the result of such reservations in class A, class B and class C, one two and three bits respectively, the decimal values of the first octet are as follows (pic. 4).

Pic. 4 - IP Adress Classes and First Octet Rule.

This way, devices could easily determine which class of IP address the deal with by looking at its first byte (Pic. 4). Knowing which class the IP address belonged to help them determine which bits described network address, and which bits denoted a host in the network (Pic. 3 highlighted in green).

Fast forward to the present day. We still use the concept of IP address classes. And as such we refer to the so called: natural network mask (length).

• Class A - Natural network mask length = 8 bits (/8) or, 255.0.0.0
• Class B - Natural network mask length = 16 bits (/16) or, 255.255.0.0
• Class C - Natural network mask length = 24bits (/24) or, 255.255.255.0

I would like to finish this post with one last observation. If your IP address uses the natural network mask length given the class (A, B, or C), we call this address a network address.

If your IP address uses the network mask length longer than the natural network mask used by this class, we call this IP address a subnet.

In my next post, I will show you how you can create subnets and how to calculate those given the host requirements.