IP options are additional fields appended to the standard 20-byte IP header. Although IP options are not required on each IP header, the ability to process IP option fields is required. IP options are used infrequently and mostly for network testing purposes.
The IP options portion size of the IP header varies in length based on the IP options that are being used. The individual IP options also vary in length from a single byte to multiple four-byte quantities. Recall that the maximum-sized IP header that can be indicated with the Header Length field is 60 bytes. With a standard IP header size of 20 bytes, 40 bytes are left for IP options.
The first byte of each IP option has the format shown in Figure 5-11.
Figure 5-11. The structure ...