Counts ICMP Address Mask Request messages received, an old mechanism by which a host asks a router for the subnet mask of its network. This is a largely obsolete request type, so seeing this counter increment on an external-facing interface is a useful indicator the gateway is being probed by reconnaissance tooling using older ICMP techniques, since modern hosts get their mask via DHCP. Example: a spike in icmpInAddrMasks from an unfamiliar external IP right before a broader port scan begins is consistent with automated recon tooling running through a checklist of ICMP probe types.