Tuesday, September 8, 2009

Public Service Announcement: Of IP and DHCP

To find your IP address and DHCP server on a PC, you bring up the Command Prompt and type "ipconfig".

To find your IP address and DHCP server on a Mac, you bring up the Terminal and type "ipconfig getpacket en0" (number 0).  There is not, so far as I can tell, any other way to make the Mac tell you the DHCP server address; although there are three different places to go for network-related information, none of them have this information.  Also, if you have manually entered an IP address for the computer to use, this command will not give you any information at all.

If you are accessing the internet across a LAN, your IP address is assigned locally, behind (?) the subnet mask, which almost always has an address of for whatever reason.  The whole network shares one IP address it gives out to the rest of the internet, which address is (obviously, probably) not the same as the one your computer spits out when you type the above commands.

Local IP addresses are assigned by your local DHCP server, which is a network application that runs on a local computer.  Which computer runs it appears to be arbitrary, and I admit I have no idea how to change that if I wanted to.  (There is a box on Macs in the Network Preferences where you can manually type in your DNS server but so far as I can tell not one for your DHCP server.  A shame since I think being able to change this might fix the network problems we're having.)

For the record, knowing this stuff probably isn't going to let you fix the problem, but it may give you an idea what the problem is so you know what to ask the real IT guys without looking dumb.  It's not going to help you explain the problem to you co-workers or boss, though, so you may still look dumb to them.

Brought to you by your local front-line tech support guru, discovered while flailing around at random because our IT contractors are out of town.

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