Client ID

Question

Level 1

5 points

Client ID

I want to know the association between IP numbers and individual devices on my local network. I am connected to my ISP (Cox) with their standard cable modem. The ethernet line from this plugs into my Time Capsule. Of course, the time capsule acts as a router on which I have established a local network. When I run AirPort Utility, click on Manual Setup, then the Advanced Button, Logs and Statistics, and finally the DHCP tab, I get a display of MAC Addresses. IP Addresses, and Lease Time for each. But the Client ID column is totally blank. The Help for AirPort Utility (DHCP -> Monitoring DHCP clients on your network) contains a cryptic footnote that "The client ID is optional and may not be required in your network." So I am left with no reasonable way to determine which local ID is associated with which device. Yes, I can go around to each device, write down its MAC address, and then figure out the IP number. I would have to repeat the process each time we've had nasty power failures as we have around here for the last week.

Please bear with me while I write my understanding of how all this works, and correct me where I'm wrong. My ISP issues single IP address for my connection. Given that, the Time Capsule via DHCP assigns a local address to each device on the local network, e.g., things like 10.0.1.200, 10.0.1.199, etc. These are strictly local, and the ISP has nothing to do with them. Every previous router I've ever owned would interrogate the individual devices to find out what they considered their names, and display these in a list along with the corresponding local IP number. But not this one. Since the Time Capsule router is setting up the local area network, I would expect it to be able control its own network, and request client IDs from the individual devices. But there seems to no way to do that.

So what gives?

John Doner

MacBook Pro, Mac OS X (10.5)

Posted on Jul 15, 2008 10:06 AM

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Answer 1

Tesserax

Level 10

158,160 points

Jul 15, 2008 10:49 AM in response to jedoner

When I run AirPort Utility, click on Manual Setup, then the Advanced Button, Logs and Statistics, and finally the DHCP tab, I get a display of MAC Addresses. IP Addresses, and Lease Time for each. But the Client ID column is totally blank.

As you already know, the Client ID or (DHCP Client ID) is an optional value. You would create these for each Apple client that you would be connecting to your network by entering a value in the "DHCP Client ID" field on the Network > Advanced > TCP/IP page from System Preferences. Thus, if you don't enter these values this would be the reason that the column would be blank.

So I am left with no reasonable way to determine which local ID is associated with which device. Yes, I can go around to each device, write down its MAC address, and then figure out the IP number. I would have to repeat the process each time we've had nasty power failures as we have around here for the last week.

Regardless of the reason, the default for most routers, not just the AirPorts, is to provide DHCP service. This is a dynamic service where the router's built-in DHCP server will dole out private IP addresses to LAN clients. For the AirPort, these would be in the range of 10.0.1.2 - 10.0.1.200. Other vendors, like Linksys, will use DHCP addresses in the 192.168.x.x range. Private IPs are not used on the Internet, only on LANs.

Because these addresses are dynamic, they will change over time based on the Lease Time value. This is also true of the IP address provided by your ISP unless you subscribe to a "business-grade" level of service, where your ISP will provide you with a static IP address at a additional cost.

My ISP issues single IP address for my connection.

This would be true if you are subscribing to a "business-grade" level of service. Most likely, you are getting "consumer-grade" instead. Over time, the IP address your ISP provides you will vary within a narrow range that your ISP has been authorized to use.

Given that, the Time Capsule via DHCP assigns a local address to each device on the local network, e.g., things like 10.0.1.200, 10.0.1.199, etc. These are strictly local, and the ISP has nothing to do with them.

Yes, this would be correct.

Every previous router I've ever owned would interrogate the individual devices to find out what they considered their names, and display these in a list along with the corresponding local IP number. But not this one.

This is a feature of NAT. Most NAT routers, including the AirPorts, will "store" the client's MAC address, and the associated DHCP-served IP address in order to communicate thru the router to the Internet ... or to another router. This is how the router knows to return a call to the Internet to the device that requested it. NAT really doesn't care about the computer's name, that's only for us humans.

If you want to be able to tell your computers apart, outside of its MAC address, just enter a DHCP Client ID value which makes sense to you.

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Answer 2

jedoner Author

Level 1

5 points

Jul 16, 2008 10:35 AM in response to Tesserax

Thanks to Tesserax for a very helpful reply!

But yes, I am well aware of the mutability of the WAN address assigned by my ISP. However, in my case it stays the same for weeks or even months at a time. Something is always connected around here, if only the TIVO. And what I really meant to say was that +at any given time+, the ISP has provided one IP address which my router shares among the devices on the local network.

You wrote that I could assign a Client ID to each device. And sure enough, I can. I have to go around to each device and do so, if the device provides some way for it. Ok, I guess that's nice. But look, most devices already have a default client ID built-in; isn't it absurd that the Time Capsule router simply ignores this information? Other routers don't, they at least give you some clue as to what the devices are short of making you go around to read the fine print on the bottom of the box to find the MAC address. For example, on my TIVO it does provide a default client ID, although apparently no way for me to alter it. I know this because with my old router (a Belkin) the TIVO always showed up with a name; TIVO followed by some meaningless numbers. Same behavior with my printer (a Brother). Only Apple devices fail to have some sort of default ID.

So basically, my gripe is that Apple routers ought to at least try to provide some visible indication more intelligible than a MAC address about what a connected device is. What's going to happen five years from now when I connect my coffee pot, toilet-seat heater, sprinkler system, heated bathroom floors, etc., all to the internet? I ought to be able to just plug them in and not have to go through configuration rigamarole.

John

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