IP Header Explained

Mininum 20 bytes, maximum 60 bytes.
Maximum size is 65,535 bytes for IP header + data.

– Version field (4 bits) for IPv4 or IPv6
– IHL or Internet Header Length (4 bits), it is the number of 32-bit words in the header
– DS field, (6 bits) or Differentiated Service
– ECN (2 bits) explicit congestion notification
– Total length (16-bit) indicates total length of IP datagram
– Identification (16 bits) – Helps identify each datagram, counter based, very imp for fragmentation.
– Flags (3 bits) – Used to indiciate fragmentation.
– Fragment offset (13 bits) – Used to indicate offset of fragmentation.
– TTL (8 bits) upper limit of routers the datagram can pass through. Normally set at 64, although 128 or 255 is also common.
– Protocol (8 bits) – Specifies protocol encapsulated. 6 is TCP and 17 is UDP.
– Header Checksum (16 bits) – Checksum of header only and not payload.
– Source IP (32 bit)
– Destination IP (32 bit)
– Options (Up to 320 bits/40 bytes)
– IP Data if any, up to 65,515 bytes

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