network issue - dhcp server keeps dishing out strange ip addresses?

Question

if i plug an ethernet cable directly through a switch and get a valid ip, yet when i plug it through a hub, i get a bogus ip. if i unplug the hub and rearrange the ethernet cables, sometimes it assigns a legit ip but then it quickly looses connection. the hub has also been working for quite a while. all these problems have occured when some new hardware was added to the network. any idea on what is the root of the issue? also, the wireless also picks up bogus ip's, and if you keep refreshing it, sometimes you'll get a legit one that works.

is this just a dhcp server issue or something else? i'm pretty sure the hub is in good working order, since there are multiple hubs that all are giving bogus ips to multiple computers, so they all couldnt have failed at the same time. also, what would you do to fix this issue? thanks

Answer 1

You might have ARP issues. That's the table routers store in memory to connect hardware (MAC) addresses to IP addresses. On most switches and routers there's a command to view and also flush the ARP table and let it rebuild. If you can't access the switch directly via some kind of control interface, you may have to reset (hard boot). I believe your common linksys has a recessed reset button, but could possibly destroy your config, so make sure you write everything down before trying that. I can't recall ever having a problem rebooting Cisco routers.



Now, if it is a DHCP problem, it sounds like the DHCP server is confused with the hub and sees it as one machine. I've never seen this happen, but seems probable. So could be a server problem, but it seems more likely that it's the hub either not relaying hardware information correctly, or the switch not updating it's ARP table fast enough.



Answer: check "ifconfig -a" on unices (I think Apple also) or "ipconfig /all" on windows to get a glimpse of mac and ip settings, then DHCP settings on the server (which may be your switch) for lease list and lease times and maximum connections, stuff like that; make sure it's doling out the correct subnet mask -- too large of a mask, the fewer addresses you acquire. Check the ARP table on any switches between you and the DHCP server. If you're connected to an ISP, then you might be only able to use one or a handful of IP addresses, unless you are using NAT.



One last thing. I highly doubt it, but it could be a clash of networks (i.e. you have neighbors somehow sharing your network and vice versa).



Hard to diagnose on this site and sorry for the overloaded answer, but good luck. I sort of went backwards; from complicated to simple, lol.

Answer 2

What kind of IP addresses, specifically?

You'll get 169.254.x.x if there's no DHCP server on the network.



If you're just using a hub or switch straight off of your cable modem, you need to get a router. These devices (hub/switch you mentioned) would work if you only had one computer on the network, but once you have more than one internet device, you will lose connectivity, unless you have a router that's doing NAT.

Answer 3

Most likely, you have more than one device on your LAN with its DHCP server enabled. You can try disconnecting devices one-by-one until you find the one that solves the problem. Then figure out how to configure that device so it's not a DHCP server.



If you know how to do it, another way to solve the problem is to capture the bogus DHCP replies and analyze the MAC address in their header. Your ARP cache will tell you the IP address that MAC address belongs to, identifying the device. (If you don't know how to do this, just use the 'disconnect one device at a time' approach.)

Answer 4

Wow, this is a tough examine, please attempt to comb the grammar up slightly, as i visit quite understand what you're asking. i imagine you propose why is it no longer speaking after it has an IP address from DHCP? relies upon, is the DHCP server direct, like contained in the workplace, or is it finished through the ISP? Is the problem that you won't be able to communicate with different individuals on the community? reason then i'd look to the subnet and make valuable the change is configured excellent(i'm assuming you're in an workplace surroundings, astounding me if it is faulty). After that, you would might want to make valuable you're growing to be a gateway out. extra tips will succeed right here.