Firmware Version supported: 4.5 and above

The firewall's DHCP server hands out IP addresses automatically to clients on a LAN interface. Static mapping (a DHCP reservation) goes one step further: it binds a specific device's MAC address to a fixed IP, so that device always receives the same address from the DHCP server.

This matters on a GajShield firewall because most policies are IP-based. A server, printer, NAS, CCTV recorder or DNAT target that picks up a different IP after a reboot would silently fall out of any firewall rule, NAT rule, or bandwidth policy scoped to its old address. Reserving its IP keeps those rules matching. (For example, the internal server behind a destination NAT (DNAT) rule should always sit on a reserved or static IP.)

This guide covers configuring the DHCP server (if you haven't already) and then adding a static mapping.

Which step do I start at? If the DHCP server is already configured on this interface, skip to Step 2. If not, start at Step 1.

Before you begin

Have ready Detail
Interface / subnet The LAN interface the DHCP server will serve, and its subnet and mask.
Gateway The firewall interface IP that clients should use as their default gateway.
DNS The DNS server(s) clients should use (often the firewall or your internal DNS / AD DNS).
Lease range A range of IPs to hand out dynamically. Keep it clear of addresses you assign manually elsewhere (servers, the gateway, other infrastructure).
Client MAC address For each reservation, the actual NIC MAC of the device. On the device: ipconfig /all (Windows) or ip link/ ifconfig (Linux/macOS).

Watch out for randomised MACs. Phones and laptops with "private"/"random" Wi‑Fi MAC enabled present a different MAC per network, which breaks reservations. Disable random MAC for that network on the client, or reserve against its real MAC.

Step 1 — Configure the DHCP server

Go to Configuration → DHCP Server → DHCP Server Config. The following page appears:

Specify the following under DHCP Server IPv4 configuration:

Field What to enter
Interface Interface on which the IPv4 DHCP server is to be configured.
Lease Range The range of IPs the server allocates to clients — Range Start to Range End.
Lease Time(optional) How long a leased IP is held before it must be renewed.
Gateway The firewall (default gateway) IP that clients use to reach other subnets.
Subnet Mask The subnet mask the server uses to determine whether an address belongs to the subnet.
DNS (optional) DNS server(s) clients use to resolve names — Primary DNS and Secondary DNS.
WINS (optional) WINS server(s) for NetBIOS name registration/resolution — Primary WINS and Secondary WINS.

Apply the changes. Once configured, start the service from Configuration → DHCP Server → Start DHCP Service for the changes to take effect.

Step 2 — Add the static mapping

Go to the Static Mapping tab under Configuration → DHCP Server → Static Mapping and add an IPv4 static mapping.

Field What to enter
Interface The IPv4 DHCP-configured interface on which to add the mapping.
MAC Address The client's MAC to bind. Format: xx:xx:xx:xx:xx:xx or xx-xx-xx-xx-xx-xx (case-insensitive).
IP Address The static IPv4 address to bind to the MAC above.
Lease Time(optional) Lease duration for this entry. If blank, the DHCP server's default applies.
Gateway (optional) Default gateway for this client. If blank, the DHCP server's default applies.
Subnet Mask(optional) Subnet mask for this client. If blank, the DHCP server's default applies.
DNS (optional) Primary / Secondary DNS for this client. If blank, the DHCP server's default applies.
WINS (optional) Primary / Secondary WINS for this client. If blank, the DHCP server's default applies.
Description(optional) A note to identify which device this MAC belongs to. Recommended — it makes the mapping table readable later.

Avoid an IP conflict. Don't reserve an address that another device already uses as a manual static IP, and make sure the reserved IP sits in the same subnet as the interface. If your deployment hands the same range out dynamically, the reservation ensures this MAC always gets this IP rather than a random one from the pool.

 

Step 3 — Start (or restart) the DHCP service

Apply everything by going to Configuration → DHCP Server → Start DHCP Service.

On the client, release and renew the lease so it picks up the reserved address immediately — ipconfig /releasethen ipconfig /renew on Windows, or reconnect the interface. Otherwise the device keeps its current lease until it next renews.

That completes static mapping on your firewall.

Verify it worked

  • The client receives the reserved IP after a release/renew (check with ipconfig / ip addr).
  • The address matches the MAC ↔ IP pair you entered in the Static Mapping tab.
  • Any firewall, NAT or bandwidth rule scoped to that IP now applies to the device consistently across reboots.

Troubleshooting

Symptom Likely cause and fix
Client doesn't get any IP DHCP service not started, or wrong interface. Confirm Step 3, and that the client is on the interface configured in Step 1.
Client gets an IP, but not the reserved one Lease not renewed yet — release/renew on the client. Confirm the MAC matches exactly(no random/private MAC) and the mapping is on the correct interface.
Reserved IP is taken by another device Another host uses that IP as a manual static. Pick a free address in the subnet, or move the conflicting device.
Two devices fighting for addresses / odd leases Another DHCP server is active on the same LAN (e.g. a consumer router). Only one DHCP server should serve a subnet.
Changes don't take effect The service wasn't restarted after editing. Re-run Start DHCP Service (Step 3).


Attached Files:
configure_DHCP_and_static_mapping_on_your_firewall.pdf