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Any CCNA+ know Class B subnetting well?

PliotronX

PliotronX

Diamond Member
Hey so I'm trying to wrap my head around IP ranges for class B (and class A for that matter) subnets. I can do class C in my sleep but the examples in the workbook are very confusing. I've got the subnets, amount of SN and hosts, and CIDR/SNM down but just when I think I have it, Packet Tracer is not liking the A/B ranges (even with nw/bc IP's into account). What's the trick?
 
Class A,B,C aren't really used today. Just refer to everything in slash notation.

Class A would be /8, B is /16, C is /24
 
it's just a pattern that changes depending on CIDR notation and which octet gets affected

/24, intervals of 1 in third octet
/23, intervals of 2 in the third octet, ex 172.16.0.0, 172.16.2.0, 172.16.4.0, and so on to 172.16.254.0
/22, intervals of 4 in the third octet, ex 172.31.0.0, 172.31.4.0, 172.31.8.0, and so on to 172.16.252.0
/21, 8
/20, 16
/19, 32
/18, 64
/17, 128
/16, intervals of 1 in second octet
/15, intervals of 2 in second octet
/14, intervals of 4 in second octet
/13, 8
/12, 16
/11, 32
/10, 64
/9, 128
/8, intervals of 1 in first octet
 
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I'm not sure what your issue is? This is the guy who's class I am in right now. Check out his video on VLSM, one of the better ones out there, and free. He has some other subnetting video's, but this is the one concerned with Class A addresses.

https://www.youtube.com/watch?v=RXKYhfIU5xk

Having a spreadsheet made up like the one in the video is a huge help.
 
Cisco still has classes in its syllabus? I thought class-based addressing went out well over a decade ago?
 
mammador said:
Cisco still has classes in its syllabus? I thought class-based addressing went out well over a decade ago?
Click to expand...

I believe they leave it in, as part of a "why we do things this way today..." background.

There are still folks around that think class is defined by the mask, and it's not. As you know, it's defined by the leading bits of the leading byte .... there are college-level instructors that don't know / don't teach that.

Cisco also uses it for some test questions (using definitions like natural mask versus the subnet mask) and how many bits are dedicated to network, subnetwork, and host portions.
 
VulgarDisplay said:
Some networks don't require anything more that RIPv1 unless you like to make things needlessly complicated.
Click to expand...

lulz, no.

If you're going to use RIP, there's no excuse not to use RIPv2, unless you have to support positively ancient equipment. And chances are, if the equipment is so old it doesn't support at least RIPv2, it probably doesn't support classless routing either.
 
VulgarDisplay said:
Some networks don't require anything more that RIPv1 unless you like to make things needlessly complicated.
Click to expand...

And if you don't mind enabling even a 5 year old to screw up your routing protocol. If "RIPv2" is complicated, it is time go take a class or exit the networking industry. Hell most of the newer gear I have been seeing doesn't even support RIPv1.

Hell OSPF and EIGRP can be set up in a simple network by just turning them on....
 
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