How to configure /31 subnet for point-to-point links

Benefits of  /31 subnets for point-to-point links

In general we use the /30 bit subnet mask for point to point networks and will get two usable host IP addresses for point to point configuration except one network and one broad address. Now we can use 31-bit subnet mask for point to point networks and it will save the IPv4 address space. In this subnet there is no unused network and broadcast address and it uses only two IP addresses: all zero bits and all one bits in the host portion of the IP address.

In /31 subnet the two IP addresses are used 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. Support for an IPv4 address with a 31-bit subnet mask is described in RFC 3021.

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.

/31 subnet effectively doubles the number of point-to-point links you can address from a finite range. Cisco IOS has supported /31 subnets for point-to-point links since release 12.2(2)T. A thorough explanation of the feature is presented here.

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 10.102.21.192/31 can be configured:
• 10.102.21.193 – Subnet for directed broadcast: {Network-number , -1}
• 10.102.21.192 – Subnet for network address: {Network-number , 0}

Example: /31 subnet configuration for point-to-point links

Router1#sh run int Gi0/1
Building configuration…

Current configuration : 137 bytes
!
interface GigabitEthernet0/1
ip address 10.102.21.192 255.255.255.254
no ip redirects
no ip proxy-arp
duplex auto
speed auto
end

Router1#sh cdp nei de

————————–
Device ID: Switch1.cisco.com
Entry address(es):
IP address: 10.102.21.193
Platform: cisco WS-C3560-12PC-S, Capabilities: Router Switch IGMP
Interface: GigabitEthernet0/1, Port ID (outgoing port): GigabitEthernet0/1
Holdtime : 145 sec
Version :
Cisco IOS Software, C3560 Software (C3560-IPBASEK9-M), Version 12.2(50)SE5, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2010 by Cisco Systems, Inc.
Compiled Tue 28-Sep-10 13:21 by prod_rel_team

advertisement version: 2
Protocol Hello: OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000FFFFFFFF010221FF0000000000002037060DBB80FF0000
VTP Management Domain: ‘paher’
Duplex: full

Router1#

Router1#ping 10.102.21.193

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.102.21.193, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Router1#

—————————————————————————–

Switch1#sh run int gi0/1
Building configuration…

Current configuration : 127 bytes
!
interface GigabitEthernet0/1
no switchport
ip address 10.102.21.193 255.255.255.254
no ip redirects
no ip proxy-arp
end

Switch1#sh cdp nei de

————————-
Device ID: Router1.cisco.com
Entry address(es):
IP address: 10.102.21.192
Platform: Cisco CISCO1941/K9, Capabilities: Router Switch IGMP
Interface: GigabitEthernet0/1, Port ID (outgoing port): GigabitEthernet0/1
Holdtime : 144 sec
Version :
Cisco IOS Software, C1900 Software (C1900-UNIVERSALK9-M), Version 15.0(1)M6, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2011 by Cisco Systems, Inc.
Compiled Wed 01-Jun-11 15:31 by prod_rel_team

advertisement version: 2
VTP Management Domain: ”
Duplex: full
Management address(es):

Switch1#ping 10.102.21.192

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.102.21.192, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/8 ms
Switch1#

Filed Under: IP Addressing

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