When I was building the Enchanted Cottage for the Enchanted Objects challenge I learnt some things about the Arduino Yún. One that puzzled me was that it mentioned that you should be able to open a browser and navigate directly to your Arduino via arduino.local
This never worked for me and I always used the IP address.
OctoPi uses something called ‘mDNS’ (also known as ‘Bonjour’ or ‘zeroconf’) to broadcast this address on the local network and make it discoverable by other PCs that understand mDNS
I recognised ZeroConf from when I was looking at the locking down the services running on the Yún so it would make sense that it's using the same mechanism. Gina also mentions that not all systems handle mDNS by default so you have to install some software if you want that to work.
How to achieve that depends on what operating system you are running. Linux users should make sure that libnss-mdns is installed. On Debian and Ubuntu, a simple sudo apt-get install libnss-mdns should take care of that.
Windows users need to install Bonjour Print Services for Windows, which can be downloaded from Apple at support.apple.com/kb/DL999. After installation, you’ll need to make sure that your Windows Firewall allows traffic on UDP port 5353 and that mDNSresponder.exe has network access.
Mac users are lucky; for them, mDNS should be supported out of the box by the operating system, without the need to install anything else.
An additional notes from the references below is that if you've install ITunes then you'll already have Bonjour installed.
So if you want to use Arduino.Local then you need to follow these instructions too.