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
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
• 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!
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