The TCP/IP Guide - IP Datagram Options and Option Format

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Table 59 lists the most common IPv4 options, showing the option class, option number and length for each (a length of 1 indicating an option that consists of only an Option Type field), and providing a brief description of how each is used.

**Table 59: Internet Protocol Version 4 (IPv4) Options**
Option Class	Option Number	Length (bytes)	Option Name	Description
0	0	1	End Of Options List	An option containing just a single zero byte, used to mark the end of a list of options.
0	1	1	No Operation	A “dummy option” used as “internal padding” to align certain options on a 32-bit boundary when required.
0	2	11	Security	An option provided for the military to indicate the security classification of IP datagrams.
0	3	Variable	Loose Source Route	One of two options for source routing of IP datagrams. See below for an explanation.
0	7	Variable	Record Route	This option allows the route used by a datagram to be recorded within the header for the datagram itself. If a source device sends a datagram with this option in it, each router that “handles” the datagram adds its IP address to this option. The recipient can then extract the list of IP addresses to see the route taken by the datagram. Note that the length of this option is set by the originating device. It cannot be enlarged as the datagram is routed, and if it “fills up” before it arrives at its destination, only a partial route will be recorded.
0	9	Variable	Strict Source Route	One of two options for source routing of IP datagrams. See below for an explanation.
2	4	Variable	Timestamp	This option is similar to the Record Route option. However, instead of each device that handles the datagram inserting its IP address into the option, it puts in a timestamp, so the recipient can see how long it took for the datagram to travel between routers. As with the Record Route option, the length of this option is set by the originating device and cannot be enlarged by intermediate devices.
2	18	12	Traceroute	Used in the enhanced implementation of the traceroute utility, as described in RFC 1393. Also see the topic on the ICMP Traceroute messages.