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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Message Types and Formats                      9  ICMP Version 4 (ICMPv4) Informational Message Types and Formats

ICMPv4 Router Advertisement and Router Solicitation Messages 1 2 ICMPv4 Traceroute Messages

ICMPv4 Address Mask Request and Address Mask Reply Message Format

The Address Mask Request and Address Mask Reply, like some other request/reply pairs, have the same basic format—the host creates the request with all fields filled in except the subnet mask value itself, and the router supplies the mask and sends the reply back to the host. The format is described in Table 100 and Figure 150.

The Identifier and Sequence Number fields can be used to match up requests and replies, as they are for Echo and Echo Reply messages. However, a host won't normally send multiple requests for subnet masks the way it might send Echo messages for testing. For this reason, the Identifier and Sequence Number fields may be ignored by some implementations.

The use of Address Mask Request and Address Mask Reply messages is optional, just as the router discovery described in the previous topic is. Other methods besides these messages or manual configuration may be used to tell a host what subnet mask to use. Again, a common alternative to ICMP for this is a host configuration protocol like the TCP/IP Dynamic Host Configuration Protocol (DHCP). Routers do have to be able to respond to Address Mask Requests for hosts that choose to send them.

ICMPv4 Router Advertisement and Router Solicitation Messages 1 2 ICMPv4 Traceroute Messages