MacOS – Multiple macs self-assigned IP on ethernet (wifi okay though)

Update: It wasn't the VPN client causing this, but after lengthy troubleshooting (see comments) it turned out to be Skype, possibly due to UPnP related bugs.

Try disabling your Cisco AnyConnect(?) VPN client (if you're not sure how, open Activity Monitor, find a process named acvpnagent and quit it).

From your description, it may well be the case that the VPN client is either failing to establish a tunnel or taking a rather long to do so, either of which could cause the apparent loss of the default route you're describing (you can ping anything on your local network, but nothing past the router).

If disabling the above (and any other) VPN client doesn't resolve it, please add the results of:

• ifconfig (not just ifconfig en1 - there's an interface utun0 appearing in your log)
• netstat -rnfinet

... at each of these stages:

• immediately after connecting to your Wifi (while the network seems to be up)
• during the problem
• and after recovery

This looks like a basic networking issue where you might need some knowledge to sniff the packets on en0 using tcpdump.

The things that are known to prevent DHCP from reaching the OS are:

1. Firewalls (OS X, Little Snitch type, malware, and/or internet firewall like Symantec and others)
2. Network setup where the DHCP server only sends addresses to machines that are cleared for the network

You seem to have done some of the easy troubleshooting, and I personally don't ever like deleting some plist but not all plist since there are many keychains, system plist, ByHost plist and others so you could actually make a problem partially deleting information on networking.

The one thing I would do is re-connect the known machine and record the IP address that gets assigned and swap in the "suspect" machine and force the IP to be the same. If you get network connectivity then you are on the right track with DHCP issues.