To conserve IPv4 address space, you can assign a 31-bit subnet mask to point-to-point networks. Support for an IPv4 address with a 31-bit subnet mask is described in RFC 3021.
With IPv4, four IP addresses with a 30-bit subnet mask are allocated on point-to-point networks. In contrast, a 31-bit subnet mask uses only two IP addresses: all zero bits and all one bits in the host portion of the IP address. The two IP addresses are interpreted as host addresses. They do not require broadcast support because any packet that is transmitted by one host is always received by the other host at the receiving end. Therefore, directed broadcast on a point-to-point interface is eliminated.
When the 31-bit subnet mask address is configured on a point-to-point link, you cannot use network addresses for broadcast purposes. For example, in an IPv4 broadcast scheme, the following subnets can be configured:
In a point-to-point link with a 31-bit subnet mask, the previous two addresses are interpreted as host addresses and packets are not rebroadcast.