Member Avatar for Grand666

Hey guys,

Let me start by saying that I'm not a computer networking expert or something, I'm studying networking from free materials I got on the net, CCNA etc.
I'm helping the lady in our town's library to make a schematic of the 11devices on the network, and to find out where all their cables go coz they have, for three years now, not had any drawings or paperwork on the network whatsoever. They have a Cisco 2800series router that connects to the T1 connection, then from their it connects to a Cisco 1811 wireless router, then to an Allied Telesyn switch, and then to a linksys switch. Now some of the computers are hard wired to the Allied Telesyn switch and Linksys switch.(they have two switches coz some computers are far from the Allied Telesyn switch, and implemented it so they just have one cable running to the linksys and then the computers near the linksys are connected to it. i guess the person who intalled it didnt want to run an additional of 6 cables all the way to the Allied Telesyn switch).

When we did an IPconfig on all the devices, we saw that all the hard wired devices have a default gateway different from the wireless ones. Is this something that you configure inside the Cisco 1811 router? To have a default gateway of lets say for the hard wired devices, and for the wireless devices? I can't seem to understand this, yet all the devices have a connection to the internet through the Cisco 1811 router. The subnet mask of the whole network is for all devices, wireless or wired. So its not subnetted then?
I hope I make sense.

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Short answer: seeing two different default gateways usually means the wired and wireless clients are on two different layer‑3 networks (two /24s) or that the wireless side is being presented as a separate gateway/NAT device — both approaches let the same physical router provide Internet access while exposing two gateway addresses. 's note about the 1811 holding multiple IPs and 's point about DHCP/ACL-based separation both fit common implementations (router subinterfaces + VLANs, or a wireless interface/AP doing its own DHCP/NAT).

Steps to verify (no router access required to gather useful evidence):

Windows (run on one wired and one wireless client)
ipconfig /all        # check IPv4 Address, Subnet Mask, Default Gateway, DHCP Server
arp -a               # map IP -> MAC
tracert 8.8.8.8      # see first hops

If router/switch access becomes available:
show ip interface brief
show running-config | section interface
show ip dhcp binding
show ip route
show mac address-table

How to interpret results: if the first three octets of client IP and its gateway match, that client is on that subnet; differing octets mean separate subnets/VLANs. The DHCP Server field in ipconfig /all identifies which device issued the address. Checking the public IP from a wired client and from a wireless client (use any “what is my IP” service) shows whether one side is being NATed separately. On the 1811 this is typically implemented with subinterfaces (VLANs) or separate interface addresses; on small sites it can also be an AP or wireless-router using NAT and its own scope. ACLs on the wireless gateway explain restricted/guest behavior.

Practical mapping tips for the AutoCAD diagram: capture a table of switch‑port → MAC → IP → hostname, label patch‑panel ports at both ends, use a tone tracer for physical cable runs, and record VLAN and DHCP scope info. Do not change configs without a backup and a maintenance window.

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Member Avatar for clandestine_j

You can definitely have multiple default gateways in a single subnet. First, are you 100% sure there is a single WAN connection? Second, you said this is a town library - do you know if they are VPN'd back to a town run datacenter or other IT department? Often this is the case for my municipal clients. It may be a matter that they are restricting, via firewall rules, what wireless clients can access. Often times when there is an open or somehow publicly accessible WiFi network we will set them up on a separate set of rules. One possible implementation of this would be:
1. Assign DHCP over the WiFi setting the default GW differently than the wired.
2. Have an access-list that match traffic to that GW
3. Filter based on that access-list

Do you have access to the router? It is all speculation at this point and without seeing the config of the router you likely won't discover more about the setup than you already know.

~j

Member Avatar for Grand666

hey clandestine,
no i dont have access to the router. all im doing is just mapping the network out on paper. drawing it on autocad and labelling the cables so the old lady knows exactly where their cables go an what is connected to it. i was just curious coz when i did an ipconfig to see who is connected where, i saw two different gateways. thanks for you answer.

Member Avatar for Sophia Sweeden

A Cisco router can, and often does, have more than one IP address. In this case it's likely that is the IP address of the 1800 series router's physical ethernet interface that plugs into the Telesyn switch. is likely the IP assigned to it's wireless service. The router has an IP address in each subnet, and both addresses point to the same router. The Cisco 1811 probably has yet a third IP address pointing to the Cisco 2800 as well. You can check this by examining the configuration file on the Cisco 1811.

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