Peter Bissmire

17-09-29

IP address masking
There are many forum queries about all-ones, aka 255.255.255.255 or /32, address masking and discussion of the matter is mostly rather confused.
The key lies in the fact that IP is a routed protocol.

IP address configuration and networking
Three items of information are normally provided to configure a TCP/IP network interface: its address, a subnet mask and a default gateway address. The mask has a most significant part, which, in its binary representation, is all ones, and a least significant part, which is all zeroes. The bitwise AND combination of address and mask yields a subnet address. Their bitwise AND NOT combination yields the address of the host within the subnet. This information is used to construct a routing (how do I get my message to its destination?) table and it is in this context that the mask takes effect.
A more detailed discussion of routing tables can be downloaded here.

A "this destination" address masked with 255.255.255.255, means this destination only. Thus, configuring a host address with this masking means "I am the only host in the subnet, indeed, I am the subnet, here I sit in glorious isolation with no-one to talk to". In this sense, a masking of 255.255.255.254 is also rather useless. This makes available two host addresses, 0 and 1. 0 is the "anywhere" address for the subnet and 1 is the subnet's broadcast address; neither is available as a host address. Moving to 255.255.255.252, we have four addresses available within the subnet: 0 for "anywhere", 1 and 2 for host addresses and 3 as the subnet broadcast address. For IP networking, this is the closest possible useful masking of a host address.

Internet access routers
It is common to find an IP address with 255.255.255.255 masking configured on a DSL router. What we should not forget here is that the WAN side of the router is not really sitting in a IP network or subnet. There is a point-to-point connection back through the DSL access multiplexer (DSLAM) to your ISP's broadband remote access server (BRAS), which performs routing, QoS and various other functions. Given the lack of any other connectivity, masking in this context is only of real significance on the BRAS, where "this address only" masking does, indeed, make sense. The only important part of the configuration on your router is the address itself. It thus knows the address to substitute for LAN-side, private addresses in packet headers (network address translation, NAT) in order to announce itself as the return destination for responses to any queries or requests that it sends out. No default gateway is required, the ISP's BRAS is the only possible immediate destination and it knows the way to the provider's Internet-side router.