Proxy ARP

Proxy ARP

“Proxy ARP” is also known as Proxy ARP.

　With Proxy ARP, a router responds to an ARP request addressed to a host with the MAC address of the router on behalf of that host. It is used in networks with hosts that do not understand (cannot configure) subnets.

It is effective in a network where the network addresses are in an inclusion relationship like the network configuration below.

*It is not possible to assign an IP address belonging to the “172.16.0.0/16” network to the F0 interface of Router_A. This is because it overlaps the IP address of the E0 interface. In the case of Cisco routers, even if you try to allocate forcibly, the router will warn you as follows.

The routers must be assigned correct IP addresses without overlapping.

Back to the story.

The real network that computer A belongs to is “172.16.1.0/24” and the network that
computer B belongs to is “172.16.2.0/24”

has become

　However, since Computer A’s subnet mask is “255.255.0.0”, Computer A misunderstands that it belongs to “172.16.0.0/16”.

in short

　The left side of the router is the “172.16.0.0/16” network and the right side of the router is “172.16.2.0/24” with the following relationship:

172.16.0.0/16 ⊃ 172.16.1.0/24

　When computer A sends a packet to computer B, computer A thinks it belongs to the same network as computer B.

　So computer A makes an ARP request for computer B’s IP address instead of the IP address of the default gateway.

　Since ARP uses broadcast, MAC addresses can be resolved within the same network (subnet), but MAC addresses of hosts in different networks (subnets) cannot be resolved.

　So, when a router with an interface with Proxy ARP enabled receives this ARP request, it will send an ARP reply packet on behalf of computer B.

As a result, computer A can communicate with computer B as if they belonged to the same network.

Back to the story.

The real network to which computer A belongs is “172.16.1.0/24”

The network to which computer B belongs is “172.16.2.0/24”

has become

　However, since Computer A’s subnet mask is “255.255.0.0”, Computer A misunderstands that it belongs to “172.16.0.0/16”.

in short

　The left side of the router is the “172.16.0.0/16” network and the right side of the router is “172.16.2.0/24” with the following relationship:

172.16.0.0/16 ⊃ 172.16.1.0/24

　When computer A sends a packet to computer B, computer A thinks it belongs to the same network as computer B.

　So computer A makes an ARP request for computer B’s IP address instead of the IP address of the default gateway.

　Since ARP uses broadcast, MAC addresses can be resolved within the same network (subnet), but MAC addresses of hosts in different networks (subnets) cannot be resolved.

　So, when a router with an interface with Proxy ARP enabled receives this ARP request, it will send an ARP reply packet on behalf of computer B.