To use a Raspberry Pi with a single Ethernet port as a firewall, you will need to create a virtual network interface using VLAN tagging. VLAN tagging is a method of adding a special tag to Ethernet frames that identifies which virtual network (VLAN) they belong to. By using VLAN tagging, you can create multiple virtual network interfaces on a single physical Ethernet port.

Here are the general steps to use a Raspberry Pi with a single Ethernet port as a firewall using OpenWRT:

• Install OpenWRT on your Raspberry Pi and configure the Ethernet port, as described earlier.
• Configure VLANs on your Raspberry Pi using the OpenWRT web interface.
• Create virtual network interfaces for each VLAN and assign them to firewall zones.
• Configure firewall rules for each zone using the OpenWRT web interface.
• Connect your devices to the appropriate VLANs and test the firewall configuration.

With this configuration, each VLAN will act as a separate network segment with its own firewall rules. This allows you to restrict traffic between different segments, and control access to your network resources.

It's worth noting that using VLAN tagging on a Raspberry Pi with a single Ethernet port can result in reduced network performance, as the processor will have to handle additional overhead for each VLAN. However, for low-to-moderate traffic loads, this should not be a significant issue.