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Configuring DHCP for Your SME Network

Once you have determined your local network IP address range, you need to decide whether or not to implement DHCP. DHCP is a network service that allows a device that is attached to the network to be allocated an IP address automatically.

The advantages of DHCP are:

If you have a device (such as a laptop) that may, at different times, be connected to different networks, then you don't want it set up with a static (unchanging) network address. A static address will mean that, without manual reconfiguration, the laptop will only be able to connect to a network that is configured to include that static address within its network subnet range, and even then there may be a problem if another device on the network has already got the same IP address.

So, for instance, the laptop has the static network address 192.168.0.10, then it will only connect to a subnet that is configured to support that address (e.g. 192.168.0.0/24). It will not connect to a network that is set up to use a different subnet (e.g 192.168.1.0/24 or 192.168.0.0/30). Similarly, if there is already a device on the network with the address 192.168.0.10, then there will be an address conflict and it is likely that neither device will work correctly.

Thus, you need to have the laptop set up to use DHCP (in windows, set "Obtain IP address automatically" within the TCP-IP properties in network connection setup).

You will need a device that provides a DHCP service. This is normally included as a service in the Internet access router - you will probably need to enable it as, by default, DHCP is often disabled.

Using DHCP for all removable devices (such as laptops, PDAs etc) ensures that each device is given a unique IP address.
You can opt to use only a small portion of your subnet for removable devices, leaving the rest available for static IP addresses for devices that never detach from the network.
Once a device has been removed from the network, the DHCP server de-allocates the IP address and so can allocate it to another device that connects. This means that you do not end up with unusable addresses

DHCP has one disadvantage: because its allocated IP addresses expire, if you leave a device with a DHCP-assigned network address connected and powered up permanently, then the DHCP server may de-allocate the address (it only detects new connections, not permanent connections) and the device will not be able to access the network. For this reason, you should use static network addresses for any devices which remain attached to the network and switched on permanently, such as networked printers and routers.

(HINT: on windows XP PCs you can refresh any DHCP-allocated address by using, from a DOS/command window, the command: IPCONFIG /RENEW. This avoids having to reboot the PC. Other versions of Windows have similar commands.)

So, to summarise:

Use a router with a DHCP service built-in.
Allocate only a part of your IP subnet to the DHCP server for dynamic addresses, leaving the rest available for static addresses. I always recommend leaving a few statics at the bottom of the subnet (say.1 to.7) and a larger block at the top (say.101 to.255) and allocate the rest (say.8 to.100) to DHCP.
Assign static IP addresses (from the blocks reserved above) to permanently-connected devices.
set all removable devices to obtain their IP addresses from the DHCP server.

IMPORTANT: Make absolutely sure that you've not allocated a static IP address within your designated DHCP dynamic address range. If you do, your network will fail mysteriously when the DHCP server tries to assign that address to a mobile device. The symptoms are very variable and you may end up much balder before you've sorted the problem!


Visit our computing and networking advice site Home and small business networking notes. This is a completely free resource site created by Kerry Anders to provide a comprehensive service to owners of home and SME networks.

More Network Troubleshooting and Support Articles:
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• Small Business Server Migration Guide
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• Incredibly Bad Network Designs That Still Work
• Rack Mount Enclosures and Server Racks
• Why Bother to Design Your SME Network?
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• How to Make a Network Cable
• Nine Tips for Designing a Small Business Network

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