Of course. Let's examine the structure of an IP header.

The Internet Protocol (IP) header is like the address label on a package. It contains all the necessary information for a packet of data to travel from its source to its destination across the internet. Every single packet traversing the network has one of these headers.

The most common version you'll see is IPv4, which has a header that is typically 20 bytes long. Let's break down what each part of this header does.

The IPv4 Header Structure

Below is an interactive diagram of an IPv4 header. You can hover over each field to get a description of its purpose and see its size.

Fragmentation: Splitting Up Packets

Sometimes, a packet is too large to travel across a particular network segment. This maximum size is called the Maximum Transmission Unit (MTU). When a packet exceeds the MTU, it must be broken down into smaller pieces, a process called fragmentation.

The Identification, Flags, and Fragment Offset fields in the IP header are crucial for this process.

• Identification: All fragments of an original packet share the same identification number so the recipient knows which pieces belong together.
• Flags: A 3-bit field where one bit indicates if more fragments are coming (MF bit) and another indicates if the packet should not be fragmented (DF bit).
• Fragment Offset: This tells the recipient how to reassemble the fragments in the correct order.

Let's simulate this. Imagine we have a 4000-byte packet that needs to pass through a network with an MTU of 1500 bytes.

A Quick Look at IPv6

As the internet grew, the 32-bit address space of IPv4 became insufficient. IPv6 was introduced to solve this and other issues. The IPv6 header is simpler and more efficient.

Here is a comparison highlighting the main differences:

• Larger Address Space: 128-bit source and destination addresses.
• Simplified Header: Fields like IHL, Header Checksum, and fragmentation controls have been removed. Fragmentation is handled by the source host, not routers.
• Flow Label: A new field to help identify packets belonging to the same traffic flow for QoS purposes.
• Extension Headers: Optional information is moved to separate "extension headers" that are placed after the main IPv6 header, making the main header a fixed, predictable size (40 bytes).

This streamlined design reduces processing overhead for routers, contributing to a more efficient network.