DHCP networks

DHCP settings can be specified separately for any IP network defined in the device. To find the parameters that define the DHCP networks, click on DHCP networks.

Network name

Select here the name of the network to which the settings are to apply. The configuration of IP networks is located in the LANconfig configuration menu under IPv4 > General > IP networks.

DHCP server enabled

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

On

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 and switched to the "Off" state.

Note: Only use this setting if you are certain that no other DHCP server is active in the LAN.

Off

DHCP server is permanently switched off.

Auto (default)

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 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 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.

Forward queries

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.

Stateless relay

The device accepts requests from DHCP clients in the local network. However, the device does not answer these requests itself, but forwards them to a central DHCP server in another network section (DHCP relay agent mode). The Stateless Relay Agent does not modify DHCP packets from the client to the server and back. In particular, unlike the Relay Agent, the DHCP server identifier is not modified.

The status of the DHCP server is available in the DHCP statistics.

Evaluate broadcast bit

Select here whether the DHCP server should evaluate the broadcast bit reported by the client. If the bit is not evaluated, all DHCP responses are sent as broadcasts.

DHCP cluster

Enable or disable the operation of a DHCP server in a cluster here.

Enabled

With cluster mode activated, the DHCP server monitors all of the ongoing DHCP negotiations in the network, and it additionally supplements its table with the stations which are registered at the other DHCP servers in the cluster. These stations are flagged as "cache" in the DHCP table.

Disabled (default)

The DHCP server manages information only for the stations connected to it.

Note: If the lease time for the information supplied by DHCP expires, the station requests a renewal from the DHCP server which supplied the original information. If the original DHCP server does not respond, the station then emits its rebinding request as a broadcast to all available DHCP servers. DHCP servers in a cluster ignore renew requests, which forces a rebinding. The resulting broadcast is used by all of the DHCP servers to update their entries for the station. The only DHCP server to answer the rebind request is the one with which the station was originally registered. If a station repeats its rebind request, the all DHCP servers in the cluster assume that the original DHCP server is no longer active in the cluster, and they respond to the request. The responses received by the station will have the same IP address, but the gateway and DNS server addresses may differ. From these responses, the station selects a new DHCP server to connect with, and it updates its gateway and DNS server (and other relevant parameters) accordingly.

Forwarding of DHCP requests

Address of 1st, 2nd 3rd and 4th Servers

Configure the IP addresses of up to four parent DHCP servers. If the network's DHCP operating mode is set to "Forward queries", the device forwards DHCP requests to these addresses.

Source address (optional)

Here you assign an optional source address to a relay agent. This address (the name of an ARF network, named loopback address) is used to forward client messages.

Put server replies into intermediate storage

If you enable this option, the device stores responses from the parent DHCP server so that it can directly respond to subsequent requests. This avoids unnecessary connections if the parent server is on a remote network.

Adapt server replies to the local network

If you enable this option, the device modifies the responses from the parent DHCP server, so as to adapt them to the local network. This means that it replaces the values for "default gateway", "DNS server" and "NBNS server".

Suppress ARP check

Before the DHCP server assigns an IP address, an ARP request is usually used to check whether the address has been assigned already. If there is no response to the ARP request within 3 seconds, the assignment goes ahead. This query is especially useful when computers are booting in normal networks that use fixed IP addresses. In a Public Spot network where, for example, a smartphone has to recognize that there is no Internet connection in order to display the login popup, this ARP request leads to an unnecessary delay. For scenarios such as this, this check can be disabled here.

Lease time of address assignments

In addition to the global default lease time configured under IPv4 > DHCPv4, it is possible to configure a lease time specifically for this DHCP network only.

Maximum lease time

Here you specify the maximum lease time that a client may request.

Default lease time

If a client requests IP-address data without specifying any particular lease time, the lease time set here is assigned to it.

Addresses for DHCP clients

First address

Here you enter the first IP address of the pool of addresses that you want to provide to DHCP clients. If you do not specify a range, the DHCP server automatically uses all of the free addresses on its own network.

Last address

Here you enter the last IP address of the pool of addresses that you want to provide to DHCP clients.

Netmask

Specify here the netmask associated with the selected address range. If no mask is entered, the device attempts to determine the netmask as far as possible from its own address and netmask.

Broadcast

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, you enter the broadcast address to be used here.

DANGER

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

Default gateway

As standard, the device issues its own IP address as the gateway address to computers making requests. If necessary, you can configure the IP address of another gateway by entering the appropriate address here.

Name server addresses

Primary/secondary DNS

Specify here the address of a name server and an alternative name server, to which DNS requests are to be forwarded. You can leave this field empty if you have an Internet provider or other remote site that automatically assigns a name server to the router when it logs in.

Primary/secondary NBNS

Specify here the address of a NetBIOS name server and an alternative NetBIOS name server, to which NBNS requests are to be forwarded. You can leave this field empty if you have an Internet provider or other remote site that automatically assigns a NetBIOS name server to the router when it logs in.

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.