If your computer accesses the Internet via a local network connection, it will have an internal (LAN / WiFi) as well as an external (Internet) IP Address. If you need to reference either of these on a Unix machine, you can do so relatively easily from the command line.

Checking your internal IP address:

To check the IP Address of your LAN / WiFi connection, ensure that Perl is installed, then run this command on the command line:

ifconfig | perl -nle '/inet [addr:]*(\S+)/ && print $1'

Explanation:

“ifconfig” will output your computer’s network interface parameters. These results will be piped into a Perl script with “perl -nle” running the specified line of code with line ending processing enabled and assuming a “while (<>) { … }” loop around the code. The “/inet [addr:]*(\S+)/ && print $1” line of code will output the IP Address section of the result of a regular expression search for lines containing “inet ” followed by an optional “addr:“.

Checking your external IP address:

To check the address of your Internet connection, ensure that cURL is installed, then run this command on the command line:

curl -s http://checkip.dyndns.org | sed 's/[a-zA-Z/<> :]//g'

Explanation:

“curl -s http://checkip.dyndns.org” will fetch the source of the specified URL without displaying a progress meter or error messages. This result will be piped into sed, and “sed ’s/[a-zA-Z/<> :]//g’” will replace all alphabetic characters, forward slashes, lesser than, greater than, space and colon characters with ” (nothing) globally (throughout the entire input, not just up to the first instance).

For more information on perl, curl and sed, use these commands:

man perl
man curl
man sed
perl --help
curl --help
sed --help

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