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Cisco IOS supports multiple IP Addresses on an interface. There will be one Primary IP Address on the interaface and multiple Secondary IP Addresses on the interface. There is no limit on the number of IP Addresses that can be configure on an interface. Configurng multiple IP Addresses on your cisco routers can sometime help where you have multiple subnets having one physical router interface.

The IP Addresses can be from different subnets and from different networks.

To configure multiple IP Addresses,

Configure primary IP Address

From the Interface configuration mode, configure the primary IP Address

ciscorouter# conf term

ciscorouter(config)# int fa0/0

ciscorouter(config-if)# ip address 172.20.20.1 255.255.255.0

That configures the primary IP Address.

Configure Secondary IP Address

Now, add the required secondary IP Addresses onto the same interface

ciscorouter(config-if)# ip address 172.20.21.1 255.255.255.0 secondary

This adds the additional IP Address onto the interface. In the command above, the keyword "secondary" at the end defines the additional IP Address as a secondary IP Address.

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13 Comments so far »

  1. by dave, on August 27 2008 @ 10:54 pm

     

    pls let me know the model #of this cisco router

    -thanks
    Dave

  2. by admin, on August 28 2008 @ 3:07 pm

     

    Not sure if I correctly remember. But, very likely on a 2811

  3. by Mustafa Gökhan Kurt, on July 15 2009 @ 8:47 am

     

    i have got a 1841 cisco router. it have got secondary interfaces. i have tried on packet tracer with same router model “1841″ but secondary is not available.

  4. by Jon M, on October 6 2009 @ 4:15 am

     

    I’ve configured several secondary IP addresses on my 1811, and for some reason only the primary is reachable from the internet. Any advice? They are all on the same subnet, and when doing a trace route on the secondary IPs it times out at the last hop before my router.

  5. by Blake Payne, on November 7 2009 @ 3:32 pm

     

    Jon M,
    It seems like maybe the packets TTL (time to live) is expiring before it gets there. Unless someone has something else, that’s all I could think of.

  6. by suhail, on January 29 2010 @ 4:40 am

     

    no answer of my qustion

  7. by boyd, on June 29 2010 @ 3:40 am

     

    What are the IP and subnet assigned to the primary and secondary address. Its possible that your secondary IP doesnt have a route to the internet or it doesnt participate on the routing protocol that you use. If that is the case, you need to setup a static route for the secondary IP.

  8. by Robinson A, on July 7 2010 @ 8:18 am

     

    Hi,

    Is it possible to Configure same IP Address in the Different interfaces on a Router.

    Thanks & Regards,
    Robinson A

  9. by Boni, on July 15 2010 @ 1:47 am

     

    Hi..

    My ISP has given me details below:
    IP: xxx.xxx.xxx.xxx
    SM: xxx.xxx.xxx.xxx
    Gateway: xxx.xxx.xxx.xxx
    DNS1: xxx.xxx.xxx.xxx
    DNS2: xxx.xxx.xxx.xxx

    How can I enter this information on a router interface?

    Regards,

  10. by Mf Moyo, on December 20 2010 @ 10:53 pm

     

    What is the effect of having many subnets on a single router interface? Does it affect browsing speed or increase latency?

  11. by Caio, on July 18 2011 @ 7:51 am

     

    Hi,

    If I use secondary IP of different subnets and in the same interface (e.g Vlan1) I use standby IP (HSRP), may I have conflict?

    Regards,

  12. by ali, on July 19 2011 @ 6:58 am

     

    Hi
    If you want assign the ip address,subnet Mask and default Gateway
    This is IP Address and Subnet Mask

    R1(config)#int g 0/1
    R1(config-if)#ip address 1.1.1.1 255.0.0.0
    R1(config-if)#exit

    Defautl Gateway is

    R1(config)#ip route 0.0.0.0 0.0.0.0 1.1.1.2

    or

    R1(config)#ip default-gateway 1.1.1.2

    Secondary IP is

    R1(config)#int G 0/1
    R1(congig-if)#ip address 1.1.1.1 255.0.0.0
    R1(config-if)#ip address 1.2.1.1 255.0.0.0 secondary
    R1(config-if)#ip address 1.3.1.1 255.0.0.0 sedondary
    R1(config-if)#ip address 1.4.1.1 255.0.0.0 secedonary
    R1(config-if)#n shutdown
    R1(config-if)#exit

    For Every Network their is a seprate gateway you need to assign,it is depend on your subnet and isp assigned Gateway.

  13. by Jesse Smith, on July 27 2011 @ 9:23 am

     

    the effect is that you can have the router accessible by multiple subsets. should not change much at all.

    if u are trying to trace using the secondary i beleave you need to change the routing table to match the secondary network number. something like

    ciscorouter# conf t
    ciscorouter(config)# int fa0/1
    ciscorouter(config-if)# ip address 192.168.1.1 255.255.255.0
    ciscorouter(config-if)# ip address 192.168.2.1 255.255.255.0 sec
    ciscorouter(config-if)# exit
    ciscorouter(config)# ip route 192.168.2.0 255.255.255.0 192.168.1.1

    that should forward traffic properly

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