Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 42)

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 configuration at the global level, or the per interface level, is not applicable
on interfaces configured with a 31-bit subnet mask IP address.
When the 31-bit subnet mask address is configured 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 configured:
• 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.
Configuring an IPv4 address with a 31-bit subnet mask
To configure an IPv4 address with a 31-bit subnet mask, enter the following commands.
You can configure 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-e1000-1/5)# 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 specifies the host address. The ip-mask variable specifies the IP network
mask. The subnet -mask-bits variable specifies the network prefix mask.
To disable configuration for an IPv4 address with a 31-bit subnet mask on any interface, use the no
form of the command.
You cannot configure 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
configured.
Error: Cannot assign /31 subnet address as secondary
Configuration example
FIGURE 2 Configured 31- bit and 24-bit subnet masks
Router A is connected to Router B as a point-to-point link with 10.1.1.0/31 subnet. There are only two
available addresses in this subnet, 10.1.1.0 on Router A and 10.1.1.1 on Router B,
Configuring an IPv4 address with a 31-bit subnet mask
42 FastIron Ethernet Switch Layer 3 Routing Configuration Guide
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