The address recorded by a router should be its outgoing interface. That is, if a router forwards a datagram that has the record route option enabled, and if the datagram arrives at the router via an interface with IP address X and leaves via an interface with IP address Y, Y should be recorded.

The format of the option is the following:

1. The ?rst byte of the option will have binary value 0000 0111. The ?rst bit indicates that the option should be copied into ONLY the ?rst fragment and NOT into any fragments after the ?rst. The next 2 bits indicate that this option is in "class 0" (i.e., a "control" option). The last 5 bits indicate that this is option 7 within class 0 (i.e., record route).

2. The second byte indicates the total number of bytes allocated in the option portion of the IP header.

3. The third byte is a "pointer." Its value indicates the next byte within the allotted space at which the next router IP address can begin. The pointer value is relative to the beginning of the option, which is considered to be byte 0. The initial value of this pointer must be 4.

4. All others bytes have value 0 until they are overwritten.