ICMPv4 Address Mask Request and Reply Messages
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When the Internet Protocol was first developed, IP addresses were based on a simple two-level structure, with a network identifier and host identifier. To provide more flexibility, a technique called subnetting was soon developed that expands the addressing scheme into a three-level structure, with each address containing a network identifier, subnet identifier and host identifier. The subnet mask is a 32-bit number that tells devices (and users) which bits are part of the subnet identifier, as opposed to the host identifier. All of this is described in considerable detail in the section on IP addressing.
To function properly in a subnetting environment, each host must know the subnet mask that corresponds to each address it is assignedwithout the mask it cannot properly interpret IP addresses. Just like determining the identity of a local router, a host can be informed of the local network's subnet mask either manually or automatically. The manual method is to simply to have the subnet mask manually assigned to each host. The automatic method makes use of a pair of ICMP messages for subnet mask determination, which were defined in RFC 950, the same standard that defined subnetting itself.
To use this method, a host sends an Address Mask Request message on the local network, usually to get a response from a router. If it knows the address of a local router it may send the request directly (unicast), but otherwise will broadcast it to any listening router. A local router (or other device) will hopefully receive this message and respond back with an Address Mask Reply containing the subnet mask for the local network. This process is somewhat similar to the mechanism used by a host to solicit a router to respond with a Router Advertisement, except that routers do not routinely send subnet mask information; it must be requested.
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