How do I find out if an IP is contactable?

If the host is not blocking ICMP echo requests (type 8, code 0) try using the "ping" command, it should work from any Unix like OS and from Windows.

UP:

C:\>ping 192.168.1.3

Pinging 192.168.1.3 with 32 bytes of data:

Reply from 192.168.1.3: bytes=32 time<10ms TTL=255
Reply from 192.168.1.3: bytes=32 time<10ms TTL=255
Reply from 192.168.1.3: bytes=32 time<10ms TTL=255
Reply from 192.168.1.3: bytes=32 time<10ms TTL=255

Ping statistics for 192.168.1.3:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\>

Not Up

C:\>ping 192.168.30.133

Pinging 192.168.30.133 with 32 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 192.168.30.133:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\>

If the host is behind a firewall blocking ICMP echo requests then you will have to look into other ways of enumerating the network, like Hping ( http://www.hping.org/ )