Add netmask writing with slashes to Networking concepts

[Chritox]

Writing the netmask as a slashed number is seen very often these days so it might be useful to include this kind of writing the netmask in the networking guide.

For example this could be named in the chapter of subnet mask
Since my English is not the best I´ll leave the writing to you.

[Tikam Singh Alma]

@BloodSKreaper Thanks and I'll do it.