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IP addressing explained

I have explained layer 4 in the previous section which is the transport layer. The packet which get created till the layer 4 will now be ready for routing. The encapsulation of layer 3 would ensure that the PDU gets routed across the internet and reaches the IP destination it is meant for. I will discuss the most popular routing encapsulation in the present date which is IPv4 in this section. I have written a separate article on IPv6 which will be on this site soon.

Let us get started with IPv4. The layer 3 or the network layer is the interfacing layer between the data link layer which is the layer 2 and the transport layer which is the layer 4. Layer 3 is of the highest significance as this is the layer which actually takes care of the routing. Layer 7 to layer 4 would come into picture only when the packet reaches the network's destination. The IP layer, data link layer and physical layer would come into picture at every routing hop. The IP layer or layer 3 is nothing but a header which gets added to the PDU formed till layer 4.

Let us discuss the IP header.
Let me discuss the various fields in the IP header. Each field would be discussed in-depth, below is just an overview of each field after that would follow an exhaustive explanation.

1) IP version :- The ip version could take several values from 0-15 out of which 4 represent IPv4. I will not be discussing any of the other values in this section. Its a four bit field with the value 0010 for IPv4 packets.

2) Header length :- This is the length of the IP header

3) ToS :- This field specifies the type of service which would be given to the IP packet

4) Total length :- This specifies the total length of the packet including the header

5) Identifier :- This is to identify the fragments when the IP packet is being re-assembled at the destination

6) Flags and Fragment offset :- Used in conjunction with the identifier to re-assemble the fragmented IP packets

7) Time to live :- Specifies the maximum number of network hops the IP packet would be allowed to be routed

8) Protocol :- Specifies the protocol which is being used the higher layer

9) Header checksum :- As the name specifies it provides the checksum of the header to ensure that the IP packet is not tampered or corrupted during the transit throught the network

10) Source Address :- Specifies the IP address of the source from the packet is originating

11) Destination Address :- Specifies the IP address of the destination for which the packet is intended

12) Options :-This field is optional and is used primarily for the purpose of testing

13) Padding :- The header needs to end on a 32 bit boundary and this is ensured by adding zeros until the lowest multiple of 32 is reached

Next section would discuss each field in detail. A comprehensive understanding of each field is a must for anyone who seeks to understand the layer 3 or IP layer in detail. Understanding the IP header would help in understanding several other routing concepts which will be explained by the articles in this site.

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