The curious case of the IP Alias

Trying to log on to Skype earlier in the week on my MacBook Pro didn’t work. For some reason it simply wouldn’t connect – it just timed out. Everything else worked absolutely fine, no issues.

Figuring it was an IPv6 issue, I unbound IPv6 from en0 and tried again. Nothing. It wasn’t my Cisco ASA firewall playing games either, although logging on to it showed a vast number of packets dropped from 192.168.1.x on its inside interface (reverse path check, I don’t use 192.168.1.x internally). How could this be?

It turns out that I had a 192.168.1.x bound to en0 from when I was testing out some locally connected kit. Skype saw this as the first IP address it could use and bound to it – whereas everything else worked fine letting the OS choose. Unbinding this address made Skype leap in to action.

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