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Two IP Addresses On Interface

Hi All,

Could you please tell me why would we use Two IP Addresses for an Interface - Primary and Secondary IP ?

Please explain scenarios, Advantages & Limitations ?

Thanks in Advance,

Prasanna Kumar Desireddy

Frequent Contributor

please see these links

please see these links

generally more disadvantages than advantages!

VIP Mentor

If your migrating to a new

If your migrating to a new subnet on your LAN  there very handy to use

VIP Expert

As Mark has noted,

As Mark has noted, secondaries can be very handy for network (prefix) migrations.

BTW, by default, Cisco devices, when using IP helpers, will only forward the primary IP.

Thanks Guys.

Thanks Guys.

Prasanna Kumar Desireddy


Cisco IOS software supports

  1. Cisco IOS software supports multiple IP addresses per interface. You can specify an unlimited number of secondary addresses. Secondary IP addresses can be used in a variety of situations. The following are the most common applications:
  2. There might not be enough host addresses for a particular network segment. For example, suppose your subnetting allows up to 254 hosts per logical subnet, but on one physical subnet you must have 300 host addresses. Using secondary IP addresses on the routers or access servers allows you to have two logical subnets using one physical subnet.
  3. Many older networks were built using Level 2 bridges, and were not subnetted. The judicious use of secondary addresses can aid in the transition to a subnetted, router-based network. Routers on an older, bridged segment can easily be made aware that many subnets are on that segment.
  4. Two subnets of a single network might otherwise be separated by another network. You can create a single network from subnets that are physically separated by another network by using a secondary address. In these instances, the first network is extended, or layered on top of the second network. Note that a subnet cannot appear on more than one active interface of the router at a time.
  5. you can use it in the lab to create multiple prefixes on loopbacks to test some features and in real world here is what Cisco says about it:
  6. With secondary addressing, multiple IP subnets coexist in the same LAN.  It's a handy tool for non-disruptively adding IP addresses to an existing LAN, without making changes to the LAN itself, and without having to renumber IP interfaces in existing subnet(s) on that LAN.
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