Cisco 1760 router Help!

[pirateclem]

I am in the midst of completing a school project and have hit a wall. Hopefully someone here can help. I have Cisco 1760 router and 2950 switch available. I have been given the better part of a class B subnet to use for this project. (net.net.35.x). My conundrum is thus: I am trying to configure the 1760 router, which has a 4 port WIC card installed. I am attempting to use all routable IP addresses in my network. I have to address A.)ethernet if 0/0 on the router B.)the VLAN which the WIC port I will be using resides within C.)a VLAN on the switch D.)2 servers which reside within the VLAN on the switch. My solution was to utilize my net.net.35.x class B subnet rather than use a NAT layer behind the router so that the servers can be directly addressed from outside of the router. I have given the if0/0 an IP address of net.net.35.11 and a subnet mask of 255.255.255.0. The WIC port I am trying to configure is if0/1 in VLAN1 on the router and have given it net.net.35.12 with a subnet mask of 255.255.255.128 However, when I attempt to set this to the router in the SDM I get an error that the IP addresses are overlapping. Any advice appreciated. I have a basic understanding and comfortability with IOS so command line answers will be appreciated as well. My main concern is understanding the problem more so than a quick fix. Thanks in advance.

 

[lanbrown]

In IP v4 addressing you have four octets (x.x.x.x) On fa 0/0 you have x.x.35.12 assigned with a subnet mask of 255.255.255.0. Just to make this easier to understand, let’s use the address of 10.10.35.12 instead. Take the subnet mask, 255.255.255.0, in relation to the 10.10.35.12, the 10.10.35 is fixed and the last octet is where addresses can be assigned for that network. In that network (Class C) you can assign 10.10.35.1-254, .0 is the network ID and .255 is the broadcast. On a separate interface, you are trying to assign 10.10.35.13 with a mask of 255.255.255.128. Those networks do overlap and that’s why you are getting an error. You cannot assign that 10.10.35 network elsewhere on that router.

You could do this, 10.10.35.12 255.255.255.128 on fa 0/0 and then 10.10.35.129 255.255.255.128 on the other interface. Those networks don’t overlap, because of the mask.

 

[Ru55ell]

You cannot have two addresses of the same network on two interfaces on the same router. That is the most simple explanation.

 

[pirateclem]

guys thanks for the quick answer. That helps alot. I did not think about the specific addresses that would fall within the subnets. I was thinking that using the extra bit of subnet mask 255.255.255.128 would create the new network. If I could, I have additional questions now that I have a bit more info. When setting up a router / switch / client such as I have described in the original problem how does one go about configuring each? ie.. The router in this case needs to be given an IP on the WAN port, an IP on VLAN1 which the WIC ports are in, the switch needs an IP for its VLAN and an IP for the client on the switch...afirmative? Or can I trunk from the VLAN on the router to the VLAN on the switch, call them the same VLAN and only have one IP for them? Hope my rambling makes sense, trying to learn here.

 

[pirateclem]

Ok, things are going better. Obviously I learned subnetting incorrectly.

I now am able to ping the servers on the switch from the router, and back etc... So the switch, servers, wic ports, router wan port can all see each other. I have to reconfigure the switch that the line from the wall is going to and will go from there. Still interested in any advice I can receive on subnetting off of my x.x.35.x Class B subnet that I have available. Seems I need the education.

 

[lanbrown]

A switch doesn't need an IP unless it's Layer-3. Layer-2 does not deal with IP addresses at all. You can assign an IP to a L-2 switch that can be managed. Don't confuse being able to manage a switch with needing an IP for it. If it's unmanaged, no IP. Next, IP routing ALWAYS deals with next hop. If you need to go from the router to the switch and they have the same IP, where's the hop?

To a previous reply you gave. You mentioned the "extra bit" of the subnet mask would separate them. Yes, that's correct, your problem was that you have a classfull network and then you tried to create a classless network within that. You can't do that.

Also, x.x.35.x is NOT a Class B network. That's a Class C.

Class A 10.x.x.x 255.0.0.0
Class B 10.10.x.x 255.255.0.0
Class C 10.10.10.x 255.255.255.0