Best way to simulate "group by" from bash?
sort ip_addresses | uniq -c
This will print the count first, but other than that it should be exactly what you want.
The quick and dirty method is as follows:
cat ip_addresses | sort -n | uniq -c
If you need to use the values in bash you can assign the whole command to a bash variable and then loop through the results.
PS
If the sort command is omitted, you will not get the correct results as uniq only looks at successive identical lines.