I have a Macbook Air 13" which I bought new in 2011 and have been using a Ethernet to USB Adapter to connect to the internet both at home and at my office. My network/IP/DNS configuration has always been 'Automatic" and for "Configure IPv4" I set it to "Using DHCP".

Today, when I took my Macbook to the office, the Ethernet to USB Adapter failed to find the correct IP and simply could not connect to the internet. When I opened Network Preference, I saw that under USB Ethernet it displayed a green light and said "connected". However on the IP address it displayed something like 169.249... (I cannot recall exactly), but one of my friend told me that typically means the system ran out of IP address due to too many users, but I knew that couldn't be truth because we have 5 IP addresses and only 3 users at the time.

My other two colleagues also happened to have the same Macbook Air and their own Ethernet-USB-Adapter, so I borrowed theirs to try on my Macbook, one managed to find IP address and I am connected instantly, the other one behaved just like mine and couldn't connect.

I went out to buy a brand new adapter, and tried it on, but it behaved just like mine and couldn't connect either. I took the adapter back to the Apple store, they did a check on it and the funny thing is it connect successfully in the store so they didn't allow me to return it. I lived in a part of the world where an adapter like this sells for $90!!!! (what a rip-off!)

I left the office dumbfounded, and I become even more dumbfounded after I got home, and when I hook up my adapter to the usual ethernet line and it connect to the internet successfully.

So basically, in my office today, I tried 4 adapters, only 1 worked. 3 didn't work in the office but they worked elsewhere.

Can anyone explain to me what's going on here?

Thank you very much.