Networking With Address Reservations

By Joseph Moran | Posted January 29, 2009

Whenever you add a new device to your network, there are two methods by which it can obtain an IP address — manually with a static address and automatically via DHCP (Dynamic Host Configuration Protocol).

Which method you choose depends on the type of device. For those you'll need consistent and reliable access to — like a printer, a NAS drive or a PC that uses port forwarding (say, for remote access from the Internet) — assigning a static address lets you decide what that address will be, secure in the knowledge that it will never change.

But configuring a device with a static address can be a pain, especially if you ever need to change it for any reason. Moreover, using static addresses gives you the added chore of having to keep track of ones being used, lest you suffer the unpleasantness that results when you inadvertently assign the same address to more than one device.

In contrast, using your router's built-in DHCP server to automatically assign a device an IP address eliminates the need to manually configure that device, and it also saves the time and hassle of having to track address usage since the DHCP server does it for you and (assuming it's working properly) knows better than to assign the same address twice. But in this scenario, DHCP — not you — decides what address a device will get and because DHCP addresses must be renewed periodically, they're not permanent and therefore will often change from time to time.

Address Reservation

The choice between DHCP and static addressing basically boils down to convenience vs. control, but with a DHCP feature called address reservation, you can have the best of both worlds: You get automatic assignment and management of IP addresses without giving up the ability to assign specific addresses set aside for the exclusive use of specific devices.

When a network device requests an IP address in a standard DHCP configuration, the DHCP server simply issues the first one available in its address pool. Later on, when the device comes back for an address renewal, it may or may not get the same address. But address reservations allow you to associate a device's unique MAC address with a particular IP address. Therefore, when that device requests an IP address, its MAC address is recognized by the DHCP server, which in turn issues the specific IP address set aside for it.

If you bought your broadband router within the past several years, there's a good chance its built-in DHCP server supports address reservations. In my (admittedly unscientific) survey of routers from three popular manufacturers — D-Link, Linksys/Cisco and Netgear, I found the feature present more often than not.

Making Reservations

To set up address reservations you'll need to log into your router and find your way to the DHCP settings area. Hint: On D-Link routers, you should find it under Setup >Network Settings; on Linksys/Cisco hardware under check SetupBasic Setup (look for a DHCP Reservation button); and on Netgear devices look under Advanced LAN Setup.

Note that menu layouts can vary by model — or even by firmware version on the same model — so your router's menus may be slightly different. If your router's DHCP server doesn't support address reservations, there's a chance, albeit slim, that updating to the latest firmware might enable the feature.

Once you've located your router's DHCP address reservation settings, you can create a reservation by providing a device's MAC address, specifying the IP address you want it to use, and giving the reservation a name. If you're not particular about which IP address is reserved for a device, you can quickly set up reservations by referring to the DHCP's table of current address assignments — highlight the device or devices you want, click Add or Reserve, and the device's current address will become a reservation. Note that some routers will let you convert assignments to reservations only one at a time.

Whether you're creating address reservations right from the address assignment list or doing it from scratch, determining a given device's MAC address may take some legwork. On Windows PCs, you can look up the MAC address by running ipconfig from a command line (the MAC address is listed under the heading Physical Address). For other network devices, like game consoles, streaming devices and set top boxes, you can usually find it via the device's menus or look it up on a sticker on the back or bottom of the device.

After you've set up reserved addresses for all your network devices, you'll find administering and troubleshooting them easier because you'll always know where they'll be located. Better yet, if you ever want to change a device's address, you can do it from the DHCP server rather than the device itself, and you won't have to worry about address conflicts the way you do with static addresses.

Adapted from PracticallyNetworked.com, part of the EarthWeb.com Network.

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