Configuring DHCP networks

The appropriate DHCP settings can be specified separately for any IP network defined in the device. You can find the parameters for defining DHCP networks in LANconfig in the configuration area "TCP/IP" on the "DHCP" tab.

When configuring DHCP networks, the addresses are defined that can be assigned to the DHCP clients (address pool). When a client is activated in the network and requests an IP address via DHCP, the device with an activated DHCP server will offer to issue an address. This address is selected from the pool of valid IP addresses. A computer which received an IP address in the past requests this address again and, assuming the DHCP server has not assigned this number to another computer in the meantime, it will attempt to issue this address again.

The DHCP server also checks the LAN to confirm that the selected address is free. Once the address is confirmed as unique, it is assigned to the requesting computer.

Note: The device factory settings include the IP networks 'Intranet' and 'DMZ', although there are no settings for IP addresses and netmasks. The device is in a special operating mode. It then uses the IP address '172.23.56.254' and the address pool '172.23.56.x' for assigning IP addresses to the network.

Note: Multiple networks on one interface: With the configuration of IP and DHCP networks, multiple networks with different DHCP settings can be active at a logical interface. In this case, the DHCP settings for the first suitable network are applied. A prioritization of networks may be necessary here.

Selecting the IP network

Select the IP network which the subsequent DHCP settings should apply to. You can find the parameters for defining DHCP networks in LANconfig in the configuration area "TCP/IP" on the "General" tab.

Enabling the DHCP server

The DHCP server can be configured to run in the following modes:

'Yes': DHCP server is permanently switched on. When this value is entered the server configuration (validity of the address pool) is checked.
- If the configuration is correct then the device starts operating as a DHCP server in the network.
- Errors in the configuration (e.g. invalid pool limits) will cause the DHCP server to be disabled.
  Note: Only use this setting if you are certain that no other DHCP server is active in the LAN.
- 'No': DHCP server is permanently switched off.
- 'Auto': With this setting, the device regularly searches the local network for other DHCP servers. The LAN-Rx/Tx LED flashes briefly when this search is in progress.
  - If another DHCP server is discovered the device switches its own DHCP server off. If the LANCOM Router is not configured with an IP address, then it switches into DHCP client mode and queries the LAN DHCP server for an IP address. This prevents unconfigured devices introduced to the network from assigning addresses unintentionally.
  - If no other DHCP server is discovered the device switches its own DHCP server on. If another DHCP server is activated later, then the DHCP server in the LANCOM Router will be disabled.
- 'Client mode': The DHCP server is disabled, the device behaves as a DHCP client and obtains its address from another DHCP server in the LAN.
Note: Only use this setting if you are certain that another DHCP server is in the LAN and actively assigning IP addresses.
'Queries forwarded': The DHCP server is active and receives requests from DHCP clients in the LAN. The device does not respond to requests itself, but forwards them to a central DHCP server in a different network segment.

The DHCP statistics show whether the DHCP server is enabled or not.

The default setting for this parameter is 'Auto'.

Assigning IP addresses

The DHCP server must first know which IP addresses it can use to assign before it can actually assign them to workstations in the network There are three different methods for selecting possible addresses:

An IP address can be taken from the defined address pool (First address: to Last address:). Any address can be entered provided it is valid for the IP network segment.
If '0.0.0.0' is entered, the DHCP server determines the relevant first and last addresses itself using the settings for the IP network (network address and netmask).
The device will be in a special operating mode if no IP network has yet been defined. It then uses the IP address '172.23.56.254' and the address pool '172.23.56.x' for assigning IP addresses to the network.

When a client is activated in the network and requests an IP address via DHCP, the device with an activated DHCP server will offer to assign an address. This address is selected from the pool of valid IP addresses. A computer which received an IP address in the past requests this address again and, assuming the DHCP server has not assigned this number to another computer in the meantime, it will attempt to issue this address again.

The DHCP server also checks the LAN to confirm that the selected address is free. Once the address is confirmed as unique, it is assigned to the requesting computer.

Assigning the netmask

The netmask is assigned in a similar way to assigning addresses. If a netmask has been entered in the DHCP settings, it will be used when assignment is made. Otherwise the IP network's netmask will be used.

Assigning the broadcast address

As a rule, broadcast packets in a local network have an address which results from the valid IP addresses and the netmask. In special cases (e.g. when using subnets for a selection of workstations) it may be necessary to use a different broadcast address. In this case the broadcast address to be used is entered in the DHCP settings.

Note: We recommend that only experienced network specialists change the pre-setting for the broadcast address. Errors in the configuration here can lead to costly connections being established!

Assigning the standard gateway

As standard, the LANCOM issues its own IP address as the gateway address to computers making requests. If necessary, the IP address of another gateway can be transmitted if a corresponding address is entered here.

Assigning DNS and NBNS servers

IP address of the DNS and NBNS name servers to which DNS and NBNS requests should be forwarded.

If no server is defined in the relevant fields, the router will forward its own IP network address as DNS or NBNS address if the DNS server has been enabled for the network in question. If the DNS server is not active for this network, then the IP address in the global TCP/IP settings is communicated as the DNS server.