As a consultant, you're often asked to design or install devices on a customer's network without complete knowledge of their IP environment. As a result, you may not know all the right parameters to configure on the new device. In this article we'll look at the basic parameters, and discuss which are actually necessary and which are not.
IP address
Obviously, the IP address is required for a host to be able to talk normally on the network. Of course, some hosts may have multiple NICs, each of which gets an IP address, and sometimes, you can assign more than one IP address to each NIC. If the customer has a trunked interface, each interface gets an IP address.
This parameter isn't one you should guess at. You could get lucky and get an unused address, or you could get unlucky and cause a duplicate IP. The worst thing that can happen is you choose the IP address of the router on the subnet, disrupting communication for everyone on the subnet.
Subnet mask
The subnet mask is absolutely necessary for each IP address you configure, and you need to know the right mask. If you guess and choose a larger or smaller address, you'll usually be able to communicate with other devices in the real subnet or the one you configured, whichever is smaller. Assuming the default gateway is in this range, you can also normally talk to devices on other networks and the Internet. However, connectivity to devices in the range between the real subnet and the one you configured will usually be disrupted.
Default gateway
The default gateway is optional. You can leave it blank if the only devices you want to talk to are on the same subnet. You can also use routes to specific other subnets you want to talk to instead of the default gateway. The easiest way to enable a host or PC to communicate to devices not on the subnet is to configure the IP address of the router on your subnet as the "default gateway." If you configure it wrong, you'll only be able to talk to the local subnet, and traffic you attempt to send to the other subnets will be sent to whatever host owns the IP address you configured as the default gateway.
DNS and WINS
DNS and WINS parameters are also optional. You need DNS to resolve hostnames to IP addresses and WINS to resolve NetBIOS names to IP addresses unless your applications directly reference IP addresses instead of names, or you statically map names to IP addresses. Especially on servers and critical systems, it's common to statically configure these in a HOSTS or LMHOSTS file to prevent DNS outages from disrupting communication. Misconfiguring these items results in lack of connectivity and sending name queries to the wrong addresses.
Of course, if DHCP is configured in the environment already, you can just enable DHCP on your device, and it will automatically configure the necessary parameters.
About the author
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years of experience in the networking industry. He is co-author of several books on networking, most recently,CCSP: Secure PIX and Secure VPN Study Guide, published by Sybex.
