Methods of Using Additional Public IP Addresses

Methods of deploying additional public IP addresses vary depending on how the addresses are delegated, the size of the allocation, and the goals for the specific network environment. To use additional public IP addresses with NAT, for example, the firewall will need Virtual IP Addresses.

There are two options for directly assigning public IP addresses to hosts: Routed public IP subnets and bridging.

Choosing between routing, bridging, and NAT

Additional public IP addresses can be put to use by directly assigning them on the systems that will use them, or by using NAT. The available options depend on how the addresses are allocated by the ISP.

Additional static IP addresses

Methods of using additional static public IP addresses vary depending on the type of assignment. Each of the common scenarios is described here.

Single IP Subnet on WAN

With a single public IP subnet on WAN, one of the public IP addresses will be on the upstream router, commonly belonging to the ISP, and another one of the IP addresses will be assigned as the WAN IP address on pfSense® software. The remaining IP addresses can be used with either NAT, bridging or a combination of the two.

To use the addresses with NAT, add Proxy ARP, IP alias or CARP type Virtual IP addresses.

To assign public IP addresses directly to hosts behind the firewall, a dedicated interface for those hosts must be bridged to WAN. When used with bridging, the hosts with the public IP addresses directly assigned must use the same default gateway as the WAN of the firewall: the upstream ISP router. This will create difficulties if the hosts with public IP addresses need to initiate connections to hosts behind other interfaces of the firewall, since the ISP gateway will not route traffic for internal subnets back to the firewall.

Figure Multiple Public IP addresses In Use Single IP Subnet shows an example of using multiple public IP addresses in a single block with a combination of NAT and bridging.


Multiple Public IP addresses In Use Single IP Subnet

See also

For information on configuration, NAT is discussed further in Network Address Translation, and bridging in Bridging.

Small WAN IP Subnet with Larger LAN IP Subnet

Some ISPs will allocate a small IP subnet as the “WAN side” assignment, sometimes called a transport or interconnect network, and route a larger “inside” subnet to the firewall. Commonly this is a /30 on the WAN side and a /29 or larger for use inside the firewall. The service provider router is assigned one end of the /30, typically the lowest IP address, and the firewall is assigned the higher IP address. The provider then routes the second subnet to the WAN IP address of the firewall. The additional IP subnet may be used by the firewall on a routed LAN or OPT interface with public IP addresses directly assigned to hosts, with NAT using Other type VIPs, or a combination of the two. Since the IP addresses are routed to the firewall, ARP is not needed so VIP entries are not necessary for use with NAT.

Because pfSense software is the gateway on the local segment, routing from the public local subnet hosts to LAN is much easier than in the bridged scenario required when using a single public IP subnet. Figure Multiple Public IP Addresses Using Two IP Subnets shows an example that combines a routed IP subnet and NAT. Routing public IP addresses is covered in Routing Public IP Addresses, and NAT in Network Address Translation.


Multiple Public IP Addresses Using Two IP Subnets

If the firewall is part of a High Availability cluster using CARP, the WAN side subnet will need to be a /29 so each firewall has its own WAN IP address plus a CARP VIP. The provider will route the larger inside subnet to the WAN CARP VIP in this type of configuration. The inside IP subnet must be routed to an IP address that is always available regardless of which firewall is up, and the smallest subnet usable with CARP is a /29. Such a setup with CARP is the same as illustrated above, with the OPT1 gateway being a CARP VIP, and the provider routing to a CARP VIP rather than the WAN IP address. CARP is covered in High Availability.

Multiple IP subnets

In other cases, a site may be allocated multiple IP subnets from the ISP. Usually when this happens, the site started with one of the two previously described arrangements, and later when requesting additional IP addresses the site was provided with an additional IP subnet. Ideally, this additional subnet will be routed to the firewall by the ISP, either to its WAN IP address in the case of a single firewall, or to a CARP VIP when using HA. If the provider refuses to route the IP subnet to the firewall, but rather routes it to their router and uses one of the IP addresses from the subnet as a gateway IP address, the firewall will need to use Proxy ARP VIPs, IP Alias VIPs, or a combination of IP Alias and CARP VIPs for the additional subnet. If at all possible, the provider should route the IP subnet to the firewall as it makes it easier to work with regardless of the firewall being used. It also eliminates the need to burn 3 IP addresses in the additional subnet, one for the network and broadcast addresses and one for the gateway IP address. With a routed subnet, the entire subnet is usable in combination with NAT.

Where the IP subnet is routed to the firewall, the scenario described in Small WAN IP Subnet with Larger LAN IP Subnet applies for an additional internal subnet. The subnet can be assigned to a new OPT interface, used it with NAT, or a combination of the two.

Additional IP Addresses via DHCP

Some ISPs require additional IP addresses to be obtained via DHCP. This is not a good means of obtaining multiple public IP addresses, and must be avoided in any serious network. A business-class connection should not require this. pfSense software is one of the few firewalls which can be used in any capacity with additional IP addresses from DHCP. This offers limited flexibility in what the firewall can do with these addresses, leaving only two feasible options.


If the additional IP addresses from DHCP must be directly assigned to the systems that will use them, bridging is the only option. Use an OPT interface bridged with WAN for these systems, and the systems must be configured to obtain their addresses using DHCP.

Pseudo multi-WAN

The only option for having the firewall pull these DHCP addresses as leases is a pseudo multi-WAN deployment. Install one network interface per public IP address, and configure each for DHCP. Plug all the interfaces into a switch between the firewall and the modem or router. Since the firewall will have multiple interfaces sharing a single broadcast domain, enable Suppress ARP messages on System > Advanced, Networking tab to eliminate ARP warnings in the system log, which are normal in this type of deployment.

The only use of multiple public IP addresses assigned in this fashion is for port forwarding. Port forwards can be used on each WAN interface that uses an IP address assigned to that interface by the ISP DHCP server. Outbound NAT to the OPT WANs will not work because of the limitation that each WAN must have a unique gateway IP address to properly direct traffic out of that WAN. This is discussed further in Multiple WAN Connections.