DNS Lookup, Reverse DNS Lookup, Domain Configuration Check and IP Address Calculators


What is DHCP

On a TCP/IP local area network, all hosts must have a unique IP address (within the LAN subnet), a LAN netmask and usually a gateway and DNS server. All of these values can be set manually on each host, but doing so requires a lot of configuration and coordination. Everytime a new host is added, it would need to be assigned a unique IP address and configured before it could participate on the LAN.

DHCP (Dynamic Host Configuration Protocol) is a protocol that can be used to automatically assign dynamic IP addresses to network hosts. It can also provide other network information like the subnet mask, the address of the gateway and address of the DNS server(s). This allows network hosts to be added or removed with minimal configuration.

There are also many other settings that can be provided by a DHCP server. These include (amongst many other things): DNS domain name, Time server, PXE Server Host Name and PXE Image File.

Configuring DHCP

The DHCP host needs to have a manually configured IP address. When you configure a DHCP server, you specify the scope. The DHCP scope determines the range of IP addresses that can be assigned. The DHCP server identifies each host by its MAC address. Each IP address is leased for a specified time period. After the time period has expired, the lease may be renewed or the IP address may be assigned to another host. DHCP servers usually allow you to reserve an IP address for a specific MAC address (if you need to). This means that the host with that MAC address will always be assigned the same IP address.

DHCP Relay

One DHCP server can be used to provide IP addresses for more than one subnet. To do this you need to use routers that support DHCP Relay and configure the scopes for each subnet on the DHCP server. A router that is configured for DHCP Relay will pass the DHCP request on to the DHCP server and then pass the DHCP server's response back to the host. DHCP Relay acks like a proxy for DHCP. The routers would have a static IP address and subnet mask and it is this that the DHCP server uses to determine which scope to assign the IP address from.

Reserved IP Address or Static IP Address

There are advantages and disadvantages with using reserved IP addresses and static IP addresses. So, if you need to have a persistant IP address, which should you use? Generally, if you are using DHCP and you have access to the DHCP server, you would use a reserved IP address. This allows you to manage all network addresses from a central location. Here are some Pros and Cons of using Reserved IP addresses against Static IP Addresses:

Reserved IP address

  • Can be done from the DHCP server with no configuration on the host.
  • The host automatically gets configured with all information from the DHCP server.
  • The host with the reserved IP address can be rebuilt and will retain the same reserved IP address automatically with no extra configuration.
  • The DHCP server will prevent the same IP address being issued to more than one host.

Static IP address

  • Requires manual configuration on the host.
  • Hosts can be replaced and the IP address can be transferred without access to the DHCP server.
  • If a host with a static IP address is rebuilt, it will need to be re-configured manually for it to use the same IP address.
  • IP address allocation will need to be recorded manually, to make sure that the same address isn't issued to more than one host.