Secondary IP address

Unanswered Question
Oct 23rd, 2008

Hi, i want to know the purpose of assigning secondary IP address to a router's interface in detail so, please let me know soon..

-Thank you,

Pradeep

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omar.elmohri Thu, 10/23/2008 - 02:37

Here is how to do that:

interface

ip address

ip address

Plese rate if that help.

Regards,

Omar

prado1985 Thu, 10/23/2008 - 03:40

Hi Omar, iam glad to see your response, actually i require to know the purpose of using secondary ip address on an interface of a router.

Thank you,

pradeep

Joseph W. Doherty Thu, 10/23/2008 - 04:45

It allows you to have multiple subnets on the same physical wire. Hosts share the same wire (such as a hub) but in different subnets would normally need to routed to communicate. Broadcasts and multicast seen by all hosts. Very handy technique when changing host network addresses, since both old and new can be on the same wire. Very weak from a security aspect, since easy to jump subnets.

With modern switches, VLANs and VLAN trunks much better way to share a switch with multiple subnets, since the VLANs will contain broadcasts and multicast. Better security too.

prado1985 Thu, 10/23/2008 - 05:24

Hi Joseph,

thank you very much i almost understood the concept, but still not getting a grip over the concept wud u plz put up a scenario where i can understand completely.

Thank you,

Pradeep

prado1985 Thu, 10/23/2008 - 06:02

Hi Joseph, I have known that Scondary IP address is asigned to carry only Updates info. and doesn't generates datagram, datagram is basically generated by primary address but wat is logic in having two address on a single interface while one address is sufficient to carry both dataram and updates info.

Thank you,

Pradeep

Joseph W. Doherty Thu, 10/23/2008 - 16:22

Usage of secondaries stems from the days before VLANs. But again, even today, secondaries are handy if you need to readdress a network segment.

Suppose you have a network segment that's 192.168.1.0/24 and you want to move all hosts on that segment to 10.1.1.0/24. So if a host is using 192.168.1.65/24 how do you readdress it to 10.1.1.65 without it losing access to the network. Normally, devices in 192.168.1.0/24 with a gateway of 192.168.1.1/24 won't be able to communicate with the router (still on the same interface) if they now are in network 10.1.1.0/24 with a gateway of 10.1.1.1/24.

Problem of keeping a viable network remains as long as you have no way of readdressing all the hosts at a specific and instance cut over time. Even DHCP hosts keep an address while the lease time is good.

Assuming your router now has an address of 192.168.1.1/24 you can add a secondary of 10.1.1.1/24 (or you make the 10.1.1.1/24 the new router address and 192.168.1.1/24 a secondary - this latter is likely better, especially for DHCP but might cause a very brief gateway interruption as you readdress). Once both addresses are active on the router interfaces, hosts in either subnet continue to work. When you've finished readdressing all the hosts, either manually, or DHCP has issued new addresses, you can remove the 192.168.1.1/24 address.

Other uses, again, were more likely in the "olde days". Assume for growth, you need to add a new class C address but you don't have a new router interface you can use or the closet contains a large chassis hub. With secondaries, you can have multiple subnets on one large chassis hub which connects to a single router interface.

sirdudesly Thu, 10/23/2008 - 04:10

http://www.cisco.com/en/US/docs/ios/12_3t/ip_addr/command/reference/ip1_i1gt.html

Scroll down a little bit

"The optional secondary keyword allows you to specify an unlimited number of secondary addresses. Secondary addresses are treated like primary addresses, except the system never generates datagrams other than routing updates with secondary source addresses. IP broadcasts and Address Resolution Protocol (ARP) requests are handled properly, as are interface routes in the IP routing table.

Secondary IP addresses can be used in a variety of situations. The following are the most common applications:

•There may not be enough host addresses for a particular network segment. For example, your subnetting allows up to 254 hosts per logical subnet, but on one physical subnet you need 300 host addresses. Using secondary IP addresses on the routers or access servers allows you to have two logical subnets using one physical subnet.

•Many older networks were built using Level 2 bridges. The judicious use of secondary addresses can aid in the transition to a subnetted, router-based network. Routers on an older, bridged segment can be easily made aware that many subnets are on that segment.

•Two subnets of a single network might otherwise be separated by another network. This situation is not permitted when subnets are in use. In these instances, the first network is extended, or layered on top of the second network using secondary addresses"

prado1985 Thu, 10/23/2008 - 05:27

Hi sirdudesly,

Many thanks for ur valuable reply..

Thank u

pradeep

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