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Routers

Posted on 2006-11-06
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Last Modified: 2013-11-29
hi there,
What is the minimum number of IP addresses that can be assigned to a router?  Do they have to be of the same class?  
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Question by:jsctechy
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12 Comments
 
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Expert Comment

by:arnold
ID: 17882978
What kind of router?  Are you talking about wan IP?  How many interfaces (serial, Ethernet, Fiber) does the router have?  All that is needed to answer your question.
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Expert Comment

by:giltjr
ID: 17883036
Two, one from each IP subnet that the router is routing between.  

If by class you mean the old out dated class A, B, C subnets, no they do not have to have the same "class" of subnet mask.
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Expert Comment

by:RPPreacher
ID: 17883144
Minimum?

Zero.  Could use IPX.  Or "unnumbered interfaces"...

Is this an academic question or do you have a specific configuration issue?
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Author Comment

by:jsctechy
ID: 17883439
well is a broad question because i did not understand it neither, i have to do a small paper where i specified the functionality of the router and the pro and cons but one of the bullets ask for a little overview of the mininum adreess a router can handle and if they have to be on the same class.
My approach to it was like it can the minimun as two and the classes thwy do not need to be the same but i get caught in the part where it says why?
Well i don't know why so i'm trying to make some sense out of it by gering a genral understanding of it.
It does not have show configuration or anything but at least that i'm able to understan the reason why it could handle differents classes.
Thanks
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Author Comment

by:jsctechy
ID: 17883453
but now you guys are making me confuse because one says 2 and the other says Zero, so whats going to be?
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Expert Comment

by:RPPreacher
ID: 17883521
Your small paper sounds like we are doing your homework.
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Author Comment

by:jsctechy
ID: 17883566
not really, i know wthe homework policy, i just want to know it because i will have a test today on it and i just want to get it over with it.
the "little paper" as you have called it is not do until the end of the year so why would i want you to do it for me. is just that i haven't research it enough to get the answer myself, the paper is more complex than what it is
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RPPreacher earned 500 total points
ID: 17883633
OK.  You said "small paper", I was using your words. ("i have to do a small paper")

In a basic sense, a router needs to have a minimum 1 IP address per interface.  A router can be configured as a router on a stick (http://www.gurulabs.com/goodies/routeronastick.php) or a single interface and thus only need 1 address.

In a more typical sense, to actually "route" between two networks (the function of a router is to route), you need 2 addresses.  Each address is in a different network and each network may be a different class (in classful networks).

In specific contradiction to this, you might set up a router with no address on either interface and use policy based routing to route based on interface name rather than on address.  So something like this "All traffic coming into interface-1 going to 128.x.x.x goes to interface-2, all other traffic goes to interface-3"  This specific exception requires no addresses.

So the answer is 2 usually, 1 sometimes, and 0 rarely.
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Expert Comment

by:giltjr
ID: 17884323
-->In a basic sense, a router needs to have a minimum 1 IP address per interface.  A router can be configured as a router on a stick (http://www.gurulabs.com/goodies/routeronastick.php) or a single interface and thus only need 1 address.

A router on a stick still needs two IP addresses (just like the reference site shows).  It is just that they are both on the same interface.

If a router need to get traffic from 10.0.1.0/24 to 10.0.2.0/24 it need to have an address on both subnets, thus two IP addresses.  No way around that, assuming you are talking about IP  routing and not bridging or some other protocol.

To route IP traffic, you must have a minimum of two IP addressed, independent of the number of interfaces you have.

I said two, because I assumed IP traffic.  However, if you are talking about non-IP based traffic or you are talking about bridging, then you don't need any.
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Expert Comment

by:RPPreacher
ID: 17884436
>A router on a stick still needs two IP addresses (just like the reference site shows).  It is just that they are both on the same interface.

I have configured a router on a stick with a single interface and a single IP.  Specifically to support this configuration,

VPN_Client-->VPN_Server-->Local_LAN-->PIX(Pre-FOS7.x)-->(L2L_VPN_Tunnel)-->Remote_LAN
                                            |
                                            ---->ROAS (router on a stick)

This was to allow a VPN client to connect to the Local_LAN and still use resources on Remote_LAN
The ROAS just provides next hop to Remote_LAN network as the PIX
Workstations in the Local_LAN use the PIX as the DGW so they do not need this; however, VPN_Clients cannot connect to Remote_LAN as their DGW is the VPN_Server.
In this case (among others) a ROAS has a since IP address and a routing table the indicates multiple gateways.

Buzz.  Oh... nice try... and remember phrase to your answer in the form of a question.
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Expert Comment

by:RPPreacher
ID: 17884471
BTW A L3 switch is a better solution but sometimes you have to work with what you got!
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Expert Comment

by:giltjr
ID: 17885128
From what you have described, your ROAS is not doing any routing, but is doing ICMP re-directs.  If this is true, then I would not really consider what you are doing routing.  But I guess technically it is.

I would have to understand more about your setup, but on the surface if all your ROAS is doing it providing the address of the real router to use, then it is unneeded.  

It seems to me that VPN server should be able to point to your PIX box as the next hop to the remote LAN.
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