There may be better solutions (at least ones closer to what you're asking for) but here's what springs to my mind:

• Configure the DHCP servers at each location to assign your MAC address a static IP, and on the laptop, just have it automatically grab the IP from DHCP. This is by far the easiest if you have router access at each place.




• The next three are essentially the same solution as each other - split the connections up:

  
  
  
  
  
  • Add and use a different network card for one location.
  
  
  • Leave USB network cards on the two ethernet cables (slower than native gigabit).
  
  
  • Use two wireless connections. Network Manager can do the hard stuff automatically so you just roam from one place to another.
  
  



• Script something for when the interface comes up. I don't know what the event would really be called - but you essentially want to know when the cable is plugged in and the whole thing is ready to go. You could just have a static setup for one network and wait for when that comes up.

  
  
  
  
  

  • Bind in using upstart if you know the connection is there on boot, or consider this Call script after connecting to a wireless network if you make the connection after boot.

  
  

  • When the network is up, do some service discovery. Is the gateway where you expect it? Are there other known servers you should be able to ping?

  
  

  • If everything is correct, leave the configuration as it is, otherwise switch to the alternative network configuration. You can either manually set this up using ifconfig or you can have two connections set up in Network Manager and just use nmcli to switch which one you're on.

  
  

Out of the three, DHCP is the easiest.