$ ping google.com
PING google.com (172.217.169.78): 56 data bytes
Request timeout for icmp_seq 0
36 bytes from vlan...(lots of stuff, not sure how sensitive).mycompany.co.uk (129.??.??.?): Communication prohibited by filter
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 5400 58db   0 0000  3d  01 a4f5 192.168.??.??  172.217.169.78 

16 bytes from ....mycompany.co.uk (192.168.???.???): Router Advertisement

Request timeout for icmp_seq 1
Request timeout for icmp_seq 2
Request timeout for icmp_seq 3
Request timeout for icmp_seq 4
Request timeout for icmp_seq 5
36 bytes from vlan.....mycompany.co.uk (129.???.??.??): Communication prohibited by filter
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 5400 6cc8   0 0000  3d  01 9108 192.168.??.???  172.217.169.78 

$ ping customhostname
PING customhostname.mycompany.co.uk (192.168.100.200): 56 data bytes

$ ping google.com
PING google.com (172.217.169.78): 56 data bytes
Request timeout for icmp_seq 0
36 bytes from vlan...(lots of stuff, not sure how sensitive).myuni.co.uk (129.??.??.?): Communication prohibited by filter
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 5400 58db   0 0000  3d  01 a4f5 192.168.??.??  172.217.169.78 

16 bytes from ....myuni.co.uk (192.168.???.???): Router Advertisement

Request timeout for icmp_seq 1
Request timeout for icmp_seq 2
Request timeout for icmp_seq 3
Request timeout for icmp_seq 4
Request timeout for icmp_seq 5
36 bytes from vlan.....myuni.co.uk (129.???.??.??): Communication prohibited by filter
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 5400 6cc8   0 0000  3d  01 9108 192.168.??.???  172.217.169.78 

$ ping customhostname
PING customhostname.myuni.co.uk (192.168.100.200): 56 data bytes

I am connected to a Wi-Fi network (which has an internet connection), and I am using a USB Ethernet adapter to connect to a second LAN (which doesn't allow internet access).

I changed my System Preferences > Network ordering so that the Wi-Fi takes precedence over the wired LAN. If I don't do this, then it seems I can't access the internet.

I am connected to a Wi-Fi network (which has an internet connection), and I am using a USB Ethernet adapter to connect to a second LAN (from which I can't figure out how to access the internet).

I changed my System Preferences > Network ordering so that the Wi-Fi takes precedence over the wired LAN. If I don't do this, then it seems I can't access the internet.

Trying to access the internet with the Service Order the other way round looks like this:

$ ping google.com
PING google.com (172.217.169.78): 56 data bytes
Request timeout for icmp_seq 0
36 bytes from vlan...(lots of stuff, not sure how sensitive).mycompany.co.uk (129.??.??.?): Communication prohibited by filter
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 5400 58db   0 0000  3d  01 a4f5 192.168.??.??  172.217.169.78 

16 bytes from ....mycompany.co.uk (192.168.???.???): Router Advertisement

Request timeout for icmp_seq 1
Request timeout for icmp_seq 2
Request timeout for icmp_seq 3
Request timeout for icmp_seq 4
Request timeout for icmp_seq 5
36 bytes from vlan.....mycompany.co.uk (129.???.??.??): Communication prohibited by filter
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 5400 6cc8   0 0000  3d  01 9108 192.168.??.???  172.217.169.78 

I've never seen this kind of ping response before and don't know what it means. It was easy enough for me to infer however that it's going to be more difficult for me to access the internet through the wired LAN.

So I rearranged the Service Order and put wifi first. Now my internet access works as usual.

MacBook Pro Mid-2015, macOS Catalina 10.15.2.

summary

I connected to wifi and a wired LAN. Wifi is the first in the Service Order in Network Preferences. There are some hostnames in the wired LAN that don't exist on the wifi.

How can I make it so that hostnames which don't exist on the wifi network, are "looked up" in the wired LAN network?

I am connected to a Wi-Fi network (which has an internet connection), and I am using a USB ethernet adapter to connect to a second LAN (which doesn't allow internet access).

I changed my System Preferences > Network ordering so that the Wi-Fi takes precedence over the wired LAN. If I don't do this, then it seems I can't access the internet.

However, that means that the custom hostnames from the wired LAN aren't available:

$ ping customhostname
ping: cannot resolve customhostname: Unknown host

if I turn Wi-Fi off then it works:

$ ping customhostname
PING customhostname.mycompany.co.uk (192.168.100.200): 56 data bytes

Additionally, I can leave Wi-Fi turned on and still access that computer via the IP:

$ ping customhostname
ping: cannot resolve customhostname: Unknown host
$ ping 192.168.100.200
PING 192.168.100.200 (192.168.100.200): 56 data bytes

But how can I make it so that hostnames which aren't recognised (in this case, customhostname) by the wifi interface, are "looked up" in the wired LAN interface?

I don't want to have to memorise the IP address, and manually update my own records whenever it changes.

MacBook Pro Mid-2015, macOS Catalina 10.15.2.

Summary

My Mac is connected to Wi-Fi and a wired LAN. Wi-Fi is the first in the Service Order in Network Preferences. There are some hostnames in the wired LAN that don't exist on the Wi-Fi.

How can I make it so that hostnames which don't exist on the Wi-Fi network, are "looked up" in the wired LAN network?

I am connected to a Wi-Fi network (which has an internet connection), and I am using a USB Ethernet adapter to connect to a second LAN (which doesn't allow internet access).

I changed my System Preferences > Network ordering so that the Wi-Fi takes precedence over the wired LAN. If I don't do this, then it seems I can't access the internet.

However, that means that the custom hostnames from the wired LAN aren't available:

$ ping customhostname
ping: cannot resolve customhostname: Unknown host

If I turn Wi-Fi off then it works:

$ ping customhostname
PING customhostname.mycompany.co.uk (192.168.100.200): 56 data bytes

Additionally, I can leave Wi-Fi turned on and still access that computer via the IP:

$ ping customhostname
ping: cannot resolve customhostname: Unknown host
$ ping 192.168.100.200
PING 192.168.100.200 (192.168.100.200): 56 data bytes

But how can I make it so that hostnames which aren't recognised (in this case, customhostname) by the Wi-Fi interface, are "looked up" in the wired LAN interface?

I don't want to have to memorise the IP address, and manually update my own records whenever it changes.