Not much to go on with your question.
Could you read through these tips (here and/or here) on asking questions and use them to help you fill in more information about what you have, what you have tried, etc.
Do you have example messages? Are the fields already broken out? Are you looking for anything in the range of the CIDR or the CIDR itself? Are you searching in the pipeline as things are processed or are you doing a general search against stored data in Elasticsearch?
Wherever you are, you could use regex, if you are in a pipeline, you can also use cidr_match() Sorry to be generic, not much to go on… 