IP Address

An IP address is a numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing.

IPv4

An IPv4 an address consists of 32 bits which limits the address space to 2³² possible unique addresses

In early stages IP address will contain two parts : network number portion and host number portion. The highest order octet (most significant eight bits) in an address was designated as the network number and the remaining bits were called the rest field or host identifier and were used for host numbering within a network.

Classful Network Architecture

n/w = network, ND = Not Defined

The number of addresses usable for addressing specific hosts in each network is always 2^N - 2, where N is the number of rest field bits, and the subtraction of 2 adjusts for the use of the all-bits-zero host portion for network address and the all-bits-one host portion as a broadcast address.

Casting

Unicast

Sending packet from one host to one particular host

Broadcast

Sending packet from one host to multiple hosts

Limited Broadcast

Broadcasting packet from one host to all other hosts in the same network (IP : 255.255.255.255)

Directed Broadcast

Broadcasting packet from one host to all hosts in other network (IP : Last IP of the network)

Subnetwork

A subnetwork or subnet is a logical subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.

Example subnet

Class C Network (200.1.2.0) Suppose this class C network need to be divided into two equal parts, Then the subnetworks will be

1. 200.1.2.0 - 200.1.2.127
2. 200.1.2.128 - 200.1.2.255

Similar to actual network, Subnetwork will also have network ID and Directed broadcast IP for then network

Routing

For a given IP address router needs to decide through which interface it should send?

For that reason we have subnet mask

Subnetmask (Network Mask)

Subnetmask contains 1's and 0's, Number of 1's represents Network ID part and subnet ID part, Number of 0's represent Host ID part

For the above example subnet mask will be 255.255.255.128

Use of it is for any given IP address if we and it with subnet mask the output will be network ID of network with which the IP belongs to

For maintaining this table router will maintain some data called routing table

Routing table for the above example

If there is more than one match then choose the one with the largest subnet mask

Similar to above example there can be variable length subnetting.

CIDR (Classless Interdomain Routing)

Instead of dividing IP addresses into classes of A, B ... we divide based on requirement. Divisions are called blocks. So for a given IP address we will not be able to fix class in classes that means we will not be able to fix network ID (Block ID) based on IP address

So Given an IP address, How are we going to decide what part of it is Block ID and what part of it is Host ID?

That is why representation of classless is a.b.c.d/n i.e, number of bits used for block ID part is 'n'

Rules for forming CIDR blocks

1. All the IP address in a block should be contiguous
2. Block size should be a power of 2
3. First IP address in block should be evenly divisible by size of the block

First IP address in the block will be reserved for block ID, Last IP address is reserved for directed broadcast address

Subnetting in CIDR

Exactly similar to classful network subnetting

Example subnet in CIDR

Consider a CIDR block 20.30.40.10/25 and we need to divide this block into two sub networks, The subnetworks will be

1. 20.30.40.0/26
2. 20.30.40.64/26

Interesting things about subnetting

For getting connected to internet we need IP address which will be provided by ISP, Along with IP ISP will also provide Default gateway (default router connected with the internet), Subnet mask (the mask you should use) and DNS (used to convert domain name into IP)

For a host, Why do we need subnet mask?

When ever you send a packet to a destination we should know

1. IP address of destination
2. Is destination in our network or in some other network

We need point 2 because if destination is in our network we can directly send packet to destination otherwise we need to send packet to router (default gateway), and router will forward it to destination.

Now how we need to decide whether destination is in our network or not, There subnet mask comes into picture

Every host will have subnet mask Host will calculate its network ID and destination network with the subnet mask and if does not match it assumes it is not in the same network and forward the packet to default gateway

In some cases we will find subnet mask as 255.255.255.255 in a host i.e, it will treat all the other IP's are present in some other network and it will forward all the packets to default gateway

Supernetting

Why do we need to do subnetting and why do we need to combine them again?

Routing table is supposed to contain one entry per network, So number of entries in routing table represents number of networks present. So if we have lots of small networks it will take longer time for router to process the table

So we will combine small networks into big network

Rules for Supernetting

1. All the networks should be contiguous
2. Size of all the networks should be same and should be a power of 2
3. The first network should be divisible by size of block

Credits

1. Wikipedia - Classful Network
2. Wikipedia - Subnetwork