The whole point is to not have to do all that extra work, because you make your input file format simple enough.
Just read a line, take the first (or only) whitespace-separated part and feed it to inet_addr() or whatever networking function you use that takes hostnames and uses them. If it's invalid, it will tell you and you can print the offending one. No need to do anything extra.
My experience has taught me that it's fragile and stupid to do any sort of "validation" by reimplementing parts of lower layers. Need IPv6 support, or hostnames instead of IPs? If you hardcoded validation for IPv4 only, then you have to do extra work to either add that duplication, or (even better) get rid of the code completely. If you let the lower layers do the work, the additional functionalty is free. (And if the data is invalid to the lower layer, it will complain for you.)
Just read a line, take the first (or only) whitespace-separated part and feed it to inet_addr() or whatever networking function you use that takes hostnames and uses them. If it's invalid, it will tell you and you can print the offending one. No need to do anything extra.
My experience has taught me that it's fragile and stupid to do any sort of "validation" by reimplementing parts of lower layers. Need IPv6 support, or hostnames instead of IPs? If you hardcoded validation for IPv4 only, then you have to do extra work to either add that duplication, or (even better) get rid of the code completely. If you let the lower layers do the work, the additional functionalty is free. (And if the data is invalid to the lower layer, it will complain for you.)