nickmcummins By default, a machine will be assigned two different IP addresses if connected via multiple interfaces (i.e. wireless and wired ethernet). Here is an example on a MacBookAir connected to the same router as such:
➜ ~ ifconfig en0: flags=8823<UP,BROADCAST,SMART,SIMPLEX,MULTICAST> mtu 1500 ether 48:d7:05:bc:91:11 nd6 options=1 media: autoselect () status: inactive en4: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500 options=10b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV> ether 98:5a:eb:d3:b9:b9 inet6 fe80::9a5a:ebff:fed3:b9b9%en4 prefixlen 64 scopeid 0x9 inet 192.168.0.113 netmask 0xffffff00 broadcast 192.168.0.255 nd6 options=1 media: autoselect (1000baseT <full-duplex,flow-control,energy-efficient-ethernet>) status: active
To change this, open System Preferences -> Network -> Thunderbolt Ethernet, and change Configure IPv4 to Using DHCP with manual address. Enter in the current IP address for the first network interface used. Then repeat the same for that interface too, so that when DHCP re-assigns these addresses (either after connection is re-initialized, or the DHCP assignment expires, if your router is not configured to assign IP addresses permanently.
If you run the ifconfig command after applying the new configurations, you should see that the IP address assigned for both interfaces is the same.
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500 ether 48:d7:05:bc:91:11 inet6 fe80::4ad7:5ff:febc:9111%en0 prefixlen 64 scopeid 0x4 inet 192.168.0.100 netmask 0xffffff00 broadcast 192.168.0.255 nd6 options=1 media: autoselect status: active en4: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500 options=10b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV> ether 98:5a:eb:d3:b9:b9 inet6 fe80::9a5a:ebff:fed3:b9b9%en4 prefixlen 64 scopeid 0x9 inet 192.168.0.100 netmask 0xffffff00 broadcast 192.168.0.255 nd6 options=1 media: autoselect (1000baseT <full-duplex,flow-control,energy-efficient-ethernet>) status: active