The ICMP information requests and reply messages were originally intended to allow a host to determine its network number. Figure shows the format for an ICMP information request and reply message.

Two type codes are available in this message. Type 15 signifies an information request message, and type 16 identifies an information reply message. This particular ICMP message type is considered obsolete. Other protocols such as BOOTP and Dynamic Host Configuration Protocol (DHCP) are now used to allow hosts to obtain their network numbers.