IP header include a variable length in which reside options. Option types are single octets with three fields:
- 1 bit copied flag, indicating whether to copy the option upon fragmentation (1 = copy)
- 2 bits of option class: 0 is control, 2 is debugging and measurement, and 1 and 3 are reserved for later use.
- 5 bits of option number, specifying the actualy option.
There are only a few IP options, as listed by
RFC 1700:
- Type 0, Number 0: End of Option List
- Type 1, Number 1: No Operation
- Type 130, Number 2: Security
- Type 131, Number 3: Loose Source Route
- Type 68, Number 4: Time Stamp
- Type 133, Number 5: Extended Security
- Type 134, Number 6: Commercial Security
- Type 7, Number 7: Record Route
- Type 136, Number 8: Stream ID
- Type 137, Number 9: Strict Source Route
- Type 10, Number 10: Experimental Measurement
- Type 11, Number 11: MTU Probe
- Type 12, Number 12: MTU Reply
- Type 205, Number 13: Experimental Flow Control
- Type 142, Number 14: Experimental Access Control
- Type 15, Number 15: ENCODE
- Type 144, Number 16: IMI Traffig Descriptor
- Type 145, Number 17: EIP
- Type 82, Number 18: Traceroute
- Type 147, Number 19: Address Extension