I suspect you'll need a USB-Ethernet adapter.
A network bridge is a device that operates at Layer-2 of the network stack, and doesn't understand the IP protocol (ie, it just forwards frames).
A router (which it sounds like you want) is Layer-3, each interface requiring an IP in a different network subnet. You then have to enable IP forwarding on the Pi, make the existing gateway aware of the new network on the far side of the Pi, update existing DHCP with a new scope, add a DHCP relay to the Pi (or add a DHCP server on the Pi, or use static to avoid altogether).
So, let's say your existing network is DHCP-enabled in the
192.168.0.0/24 subnet, and the current default gateway (your Internet gateway) is
- configure the Pi's existing interface to a static IP (eg:
- add a new Ethernet interface to the Pi (USB dongle or the like)
- configure the new interface on a new subnet (
192.168.20.0/24) with an IP of
enable IP-forwarding on the Pi:
sudo echo 1 > /proc/sys/net/ipv4/ip_forward
make the forwarding permanent. Edit the
/etc/sysctl.conf file with
sudo, and uncomment the following line:
on your existing Internet gateway device, add a new route (this will be device specific). The new route will be pointing
192.168.0.2 (the Pi's 'external' interface address)
- now, you'll configure your computers behind the Pi with an address in the
192.168.20.0/24 address space, with a default gateway of
192.168.20.1. The DNS servers on the client should point to the IP of the Internet gateway device (
192.168.1.1) if it does DNS, if not, the IPs of your ISP DNS servers (you could set up a DNS server on the Pi, if you wanted to)
- if you want to have DHCP available instead of static addresses on the internal side of the Pi, you'll have to either configure your Pi as a DHCP server and create a new scope with the new subnet, or add a new scope to your Internet gateway DHCP server if this feature is available, and using
dhcrelay on the Pi, relay DHCP requests from the inside Pi network to