Filtering lines in a text file

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grep "^10." ip.txt?

If you are using Windows and the file is not very big, export it to Excel and there are already sort and filter.

Use regular expressions if you are familiar with them.
If you are interested in a software implementation, it all depends on your programming language.

grep (or findstr under Windows)

Suggest to use perl, though, and in any language like C++ it would be a few lines

cat + grep?