Learn network fundamentals to have cutting-edge skills and hands-on experience to excel in any DevOps role.

DevOps for Developers

Learn network fundamentals essential for any DevOps specialist. You'll explore socket programming, command-line tools, OSI layer protocols, and more.

Network Fundamentals

The IPv4 packet format was defined in [RFC 791](http://tools.ietf.org/html/rfc791.html). Apart from a few clarifications and some backward compatibility changes, the IPv4 packet format did not change significantly since the publication of [RFC 791](http://tools.ietf.org/html/rfc791.html). All IPv4 packets use a 20-byte header as shown below. Some IPv4 packets contain an optional header extension that is described later.

# IPv4 Packet Header

# Fields of The Header

The main fields of the IPv4 header are:

- A 4 bit **version** that indicates the version of IP used to build the header. Using a version field in the header allows the network layer protocol to evolve.
- A 4 bit **IP Header Length (IHL)** that indicates the length of the IP header in 32-bit words. This field allows IPv4 to use options if required, but as it is encoded as a 4 bits field, the IPv4 header cannot be longer than 64 bytes.
- An 8 bit **DS field** that is used for Quality of Service.
- A 16 bit **length field** that indicates the total length of the entire IPv4 packet (header and payload) in bytes. This implies that an IPv4 packet cannot be longer than 65535 bytes.
- **Identification** every packet has an identification number which is useful when reassembling and fragmenting a packet.
- **Flags**. There are three flags in IP headers. We'll discuss their usage in the [next lesson](https://www.educative.io/courses/grokking-computer-networking/the-internet-protocol-ipv4-packet-fragmentation-reassembly):
    - Don't Fragment
    - More Fragments
    - Reserved (must be zero)
- **Fragment Offset**: This is useful when reassembling a packet from its fragments. More details can be found in the [next lesson](https://www.educative.io/courses/grokking-computer-networking/the-internet-protocol-ipv4-packet-fragmentation-reassembly).
- **Time To Live**: This number is decremented at each hop. When it becomes 0, the packet is considered to have been in the network for too long and is dropped.
-  An 8 bits **Protocol field** that indicates the transport layer protocol that must process the packet’s payload at the destination. Common values for this field  are 6 for TCP and 17 for UDP.
- A 16 bit **checksum** that protects only the IPv4 header against transmission errors.
-  A 32 bit **source address field** that contains the IPv4 address of the source host.
- A 32 bit **destination address field** that contains the IPv4 address of the destination host.
- **Options** this field is not used very often. It's often used to test out experimental features.
- **IP Data**: They payload. This payload is not part of the checksum.

The other fields of the IPv4 header are used for very specific purposes. We'll look at a few in this lesson. 

Now that we have clarified the allocation of IPv4 addresses and the utilization of the longest prefix match to forward IPv4 packets, we can have a more detailed look at IPv4 by starting with the format of the IPv4 packets.

Before We Begin

Internet and the OSI Model

The Application Layer

The Transport Layer

An Introduction to Socket Programming with Python

The Network Layer

The Link Layer

Conclusion

The Internet Protocol: IPV4 Packets

IPv4 Packet Header

Fields of The Header