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sed 's/\([0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\)\.\([0-9][0-9]*\)/\1:\2/g' |
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omg your regex is ugly.
sed -r 's/([0-9]+\.[0-9]+\.[0-9]+\.[0-9]+)\.([0-9]+)/\1:\2/g' |
And I'm pretty sure there is better.
But sed -r is not available on all sed commands. I wanted something portable. Granted, I have no example of a sed command that presently lacks it. I just know that extended regular expression support was added (to sed and anything else) sometime after such commands first existed, so I studiously avoid using extended regular expressions in situations where portability to unknown runtime environments is desired.
If one uses sed -r, then I suppose a better (well, shorter anyway) expression would be:
sed -r 's/(([0-9]+\.){3}[0-9])\.([0-9])/\1:\3/g'