To conserve IPv4 address space, a 31-bit subnet mask can be assigned 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, and 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.
IP-directed broadcast CLI conguration at the global level, or the per interface level, is not applicable on interfaces congured with a 31-
bit subnet mask IP address.
When the 31-bit subnet mask address is congured on a point-to-point link, using network addresses for broadcast purposes is not
allowed. For example, in an IPV4 broadcast scheme, the following subnets can be congured:
• 10.10.10.1 - Subnet for directed broadcast: {Network-number , -1}
• 10.10.10.0 - Subnet for network address: {Network-number , 0}
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.
Conguring an IPv4 address with a 31-bit subnet mask
To
congure an IPv4 address with a 31-bit subnet mask, enter the following commands.
You can congure an IPv4 address with a 31-bit subnet mask on any interface (for example, Ethernet, loopback, VE, or tunnel interfaces).
device(config)# interface ethernet 1/1/5
device(config-if-e1000-1/1/5)# ip address 10.9.9.9 255.255.255.254
You can also enter the IP address and mask in the Classless Inter-domain Routing (CIDR) format, as follows.
device(config-if-e1000-1/1/5)# ip address 10.9.9.9/31
Syntax: [no] ip address ip-address ip-mask
Syntax: [no] ip address ip-address/subnet-mask-bits
The ip-address variable species the host address. The ip-mask variable species the IP network mask. The subnet -mask-bits variable
species the network prex mask.
To disable conguration for an IPv4 address with a 31-bit subnet mask on any interface, use the no form of the command.
You cannot congure a secondary IPv4 address with a 31-bit subnet mask on any interface. The following error message is displayed
when a secondary IPv4 address with a 31-bit subnet mask is congured.
Error: Cannot assign /31 subnet address as secondary
Conguring IP parameters - Layer 3 switches
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04 41
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