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Identifying an existing (but unknown) static IP

lcs0500 asked
Medium Priority
Last Modified: 2008-01-09
My client has a 24 port Linksys switch that is said to have static IPs assigned to each port.  How can I connect to one of those ports and identify the assigned IP.  I have the first three octets but need the fourth.
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Who setup their router?  You should also be able to Telnet or use a Web interface to login and configure ports.


It's a quasi-public switch in a multi-occupancy building and we don't have access to the network per se.  The prior IT person didn't map the ports on the switch for the owner and we need to serve new tenants with static IPs.

We're trying to find out what IPs are assigned to each port and we don't (as of right now) have access to the cisco 2620 feeding the switch.

I know the range of the IPs but I don't know which one is on which port.  Any other suggetions?
Greg GrothComputer Network Specialist

Your going to need to do a factory reset, if you can't get the ex-installer to assist you...  
What do you mean you don't have access to the cisco feeding the switch?
Do you mean you have access to the patch panel but not the actual switch?

Am I missing something here?  How can ports be assigned to IPs on a regular switch?  It would have to be a Layer 3 Multilayer Switch in order to assign IP (which is a layer 3 protocol) to an interface.  Do you mean that certain ports are assigned to specific VLANs, and then DHCP has IP reservations by MAC address on your DHCP Server?  Sorry if I've confused you more, but I'd like to help and so far I don't understand what's going on.
Greg GrothComputer Network Specialist
I am guessing your in a subnet and you probably need to find out which one you are in.  Per the above post I am thinking it isn't by port but you only have a set group available.
What are the actual (the part you know)IP, subnet, gateway numbers?
Dean ChafeeIT/InfoSec Manager
I'm with ctrost... I don't know of a switch that you statically assign IPs to the ports. It would help to know the exact make and model of the switch, but my guess is that each client/tenant has it statically assigned at their end. can you gain access to one of the tenant's machine to check there?

You can assign IPs to specific ports on Multilayer Switches by typing "no switchport" on the interface, then assigning it an ip address with the command "ip address <ip> <subnet mask>".  However, I'm not aware of any Linksys multi-layer switches, so its more likely that what is happening here is that VLANs are configured on the router and assigned on the switchports.

For example, VLAN 5 is configured on the router with the following syntax:

   encapsulation dot1q 5
   ip address
   ip helper-address  

In this example would be the IP of the DHCP server.  Then you would have your switch configured like so:

  switchport mode access
  switchport access vlan 5
  switchport mode access
  switchport access vlan 5

In each interface, you would type "switchport mode access" to designate that port as a switchport.  Your other option would be "switchport mode trunk" in the event of that interface being used to directly connect two switches for example (in which case you would use a cross-over cable).

What happens in the above setup, is that if you plugged a PC into port 1 or 2 on that switch, it would get an IP of 192.168.5.X.  In order for those PCs to get IPs on that correct subnet, you would have to have a DHCP scope configured in DHCP for the subnet, and put the mac address of the PC into that scope to allow the PC to get an IP in that subnet.
"that is said to have static IPs assigned to each port."

Whoever "said" this is wrong.  There is no such thing as a "static IP" attached to each router port.  The person that said this is totally ignorant of routers or switches. that is NOT the way they work.  The switch or router does not assign ANY IP address to any port on the router.  Get the facts straight.


Sorry to have been away from the computer for the last several hours.  Let me try to clear up the confusion.
There is a T-1 hooked to a Cisco 2620 feeding a Cisco 2950 which feeds a second Cisco 2950 which feeds a Linksys EF2H24 24Port EtherFast II 10 100 Auto-Sensing Hub.
The 2620 is "not available" because we don't have the password yet and we can't just shut down and start over because the system serves over 65 different companies.  The owner has been kept in the dark by his ex IT guy and hasn't a clue about what is going on.  Right now, I don't have much of a clue either but I'm working on changing that.
I've been out there tonight mapping all the feeds and at least know a little bit more about how everything is hooked up.
The 2950 ports are configured as DHCP servers with a kinda-sorta format of:
  port 2 -- Gateway:  IP: 192.168.2.xx
  port 3 -- Gateway:  IP: 192.168.3.xx
The pattern varies a little, particularly with regard to the fourth octet but I mention this only to present a more clear picture of the whole.  Since the 2950 ports are all configured as DHCP servers, it's easy to figure out what's going on with them.

But, the Linksys hub serves 23 companies with static public IPs within to / subnet through are used for DHCP via the two 2950s.

At some point in the future, we'll get access to the 2600 with help from someone who speaks fluent Cisco but right now I need to identify the fourth octet of the IP at each port on the Linksys hub and if possible feed additional IPs in that range to an available Linksys 4124 switch for a few more public IPs that are needed.

To caddlady - Now that I know their general range and WHERE the public IPs are, I might be able to use the IP scanner you suggested because I've found out what two of the IPs are.

To gg711 - A reset is out of the question because so many companies are live on the system.

To ctrost - Sorry for the confusion.  Hope the above helps settle the dust.

To FixingStuff - I don't speak Cisco but my 45 years of experience in technology suggests that there should be some way to devine what I need to know without having to get into the 2600.  I hope that I'm not wrong about that and that you folks can help me get it done.  The guy that confingured all of this is Cisco-brilliant... just a tad unprincipaled (IMHO)

To scrathcyboy - Actually the statement was: "My client has a 24 port Linksys switch that is said to have static IPs assigned to each port."  With a more careful reading you will note that there is no reference to any "router port" as you suggest.  In the late 1950's when I founded my first electronic research and development company, I too thought I knew everything but since then I've figured out that I've still got a lot to learn.  In that regard, I mistakenly called the Linksys box a switch when in fact, that particular model is a hub.
I'll try to do better with the facts in the future scrathcyboy.  Thanks for the suggestion and have a wonderful weekend.

DHCP cannot be done on a switch, so its either being done on the router, or on a DHCP server.  I don't know what you mean when you say that switchports are configured as DHCP servers.  If you can get access to the hub and login to it, and if it has Cisco IOS on it, you can do a "show cdp neighbor detail" and get the IP of every device plugged into the hub.  But I don't know much at all about Linksys Hubs being used on a corporate level so that may not be possible.


There is no question that most, if not all, of the configuration is being done on/from/through the Cisco 2620 because among other things, there is no other device available in the system that could be the DHCP server.  It's unusual in my experience to see the various DCHP ports (the 2950s in this case) each having different gateway addresses but I suppose that that approach would cause each office to be reassigned the same IP after a power failure or computer having been off for some time.  The way I understand it is that if they were all using the same gateway, they would most likely wind up with a different IP.  Seems like an effective if somewhat complex approach.
Ok man, here's how I think it is setup.  The 2620 has a serial interface for the T1 with a static IP assigned to it.  Then the router will have an Ethernet Interface (probably FA0/0 or FA0/1) with another IP (internal iP like 172.* or 10.*, maybe even 192.168.*).  The ethernet Interface is connected to your 2950 Switch, and you have the Hub connected to one of the 2950 switchports, maybe even trunked to a Gig port.  Can you get in your switch and look at the config?  I'm willing to bet almost 100% that your switch has VLANs setup.  In this situation, it seems as though the VLAN subinterfaces on your router are whats acting as the gateway.  For example, on your router, would be the FA0/0.3 subinterface on the router, hence that IP would be the gateway for VLAN 3.

Get in the 2950 and post some of the config.


It is extremely frustrating to get in the middle of something like this when it is not really my area of speciality.  However, I've finally figured out WHAT is happening even if I have no idea how.  The how I'll leave up to you far more capable networking types.

It turns out that ANY port on the 2950 switches, the Linksys hub or the Linksys switch at the end of the line provides access to any of the Nated IPs (192.168.xxx.xxx) OR any one of a number of PUBLIC IPs.  But, all of the 192.168 addresses are defacto static because their DHCP range is normally one address.

Now, it turns out that the information I had was not accurate and so I asked a question here that may not even have an answer.  However, your comments helped me think through the situation and at least figure out what is going on so I'm going to split that points in some way that I hope you think is fair.

Thanks for your help.

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