[Day 9] 8. IPv4 Header

Breakdown of an IPv4 Header (20 bytes)

1. Version (4 bits):

   • Specifies the IP version (IPv4 = 4, IPv6 = 6).
   • Size: 4 bits.

2. IHL (Internet Header Length) (4 bits):

   • Indicates the length of the IP header in 32-bit words. The minimum value is 5 (for 20 bytes).
   • Size: 4 bits.

3. Type of Service (TOS) (8 bits):

   • Used for QoS (Quality of Service), specifying priority and handling.
   • Size: 1 byte.

4. Total Length (16 bits):

   • Specifies the total length of the IP packet (header + data).
   • Size: 2 bytes.

5. Identification (16 bits):

   • A unique identifier for fragmentation, used to reassemble fragmented packets.
   • Size: 2 bytes.

6. Flags (3 bits):

   • Controls fragmentation. The most significant bit is reserved, and the other two bits are used for fragmentation control.
   • Size: 3 bits.

7. Fragment Offset (13 bits):

   • Indicates the position of the fragment’s data in the original packet.
   • Size: 2 bytes.

8. Time to Live (TTL) (8 bits):

   • Limits the lifespan of a packet to prevent it from circulating forever in the network.
   • Size: 1 byte.

9. Protocol (8 bits):

   • Indicates the protocol used in the data portion (e.g., 6 for TCP, 17 for UDP).
   • Size: 1 byte.

10. Header Checksum (16 bits):

    • Used for error-checking the header.
    • Size: 2 bytes.

11. Source IP Address (32 bits):

    • The IP address of the sender.
    • Size: 4 bytes.

12. Destination IP Address (32 bits):

    • The IP address of the receiver.
    • Size: 4 bytes.