Operation Manual

Appendix B: Types of ICMP Messages
95
The Information Request and Information Reply ICMP messages are obsolete. They
were used earlier by network nodes to determine their inter-network addresses, but are now
considered outdated and should not be used.
The Address Mask Request and Address Mask Reply ICMP messages are used to find
out the mask of a subnet (i.e. what address bits define a network address). A local node
sends an Address Mask Request to a gateway and receives an Address Mask Reply in
answer.