BSIG: IP Datagram Fields - IP Header (green) & Data (Black)

4 Bits 4 Bits 8 Bits 16 Bits 16 Bits 3 Bits 13 Bits
VERS HLEN Type of
Service		 Total
Length		 Identification Flags Frag
Offset

8 Bits 8 Bits 16 Bits 32 Bits 32 Bits var -
TTL Protocol Header
Checksum		 Source IP
Address		 Destintation
IP Address		 IP
Options		 Data ...

  • VERS - Version number
  • HLEN - Header length, in 32 bit words
  • Type of Service - How the datagram should be handled
  • Total Length - Total length (header + data)
  • Identification, Flags, Frag Offset - Provides fragmentation of datagrams to allow differing MTUs* in the internetwork
  • TTL - Time-To-Live
  • Protocol - The upper-layer (Transport Layer) protocol sending and receiving the datagram. (6 = TCP, 17 = UDP)
  • Header Checksum - Integrity check on the header.
  • Source IP Address and Destination IP Address - 32-bit IP addresses.
  • IP Options ** - Network testing, debugging, security, and other options.

* MTUs - Maximum Transmission Unit. The maximum packet size, in bytes, that a particular interface can handle.
** The IP header is variable in length because of the IP Options field.

Important Note: IP Datagrams are tagged with MAC (Media Access Control) layer headers and trailers.

BSIG: IP Datagram Fields - IP Header (green) & Data (Black)