Class A and B subnetting

FluxCapacitor

I am preparing for the ICND1 exam. I am reading the sybex book by Todd Lamle and I was wondering if I should expect to see subnetting questions on class A and B networks because I can subnet class C networks just fine but when I get to class B where the numbers become huge I struggle. Will most of the subnetting questions be on class C? What should I expect to see on the exam as far as class goes?

Thanx

luke_bibby

You should assume the worst and expect to see class A, B, and C, subnetting questions on the exam

If you understand the concept of subnetting then with enough practice you will be proficient subnetting any classful network. Reread the part on block sizes (or magic numbers - can't remember which term Lammle uses) and practice practice practice practice

Kaminsky

FluxCapacitor wrote: »

I am preparing for the ICND1 exam. I am reading the sybex book by Todd Lamle and I was wondering if I should expect to see subnetting questions on class A and B networks because I can subnet class C networks just fine but when I get to class B where the numbers become huge I struggle. Will most of the subnetting questions be on class C? What should I expect to see on the exam as far as class goes?

Thanx

I know it sounds odd but if you can subnet class C, you can subnet class A & B. You just have to keep a firm grasp of the rules, what the subnet is and how it increments in your mind.

/27 for instance. The subnet will be:

32 for a class C. Will increment 32, 64, 96, etc.
0.32 for a class B. Will increment 0.32, 0.64, 0.96, etc.
0.0.32 for a class A. Will increment 0.0.32, 0.0.64, 0.0.96, etc.

Yes the number of subnets and hosts can get much larger but just use the same maths as you do for the class C

dynamik

Just remember that the number of hosts/networks doubles as you add bits and halves as you subtract bits.

So if you know that 10 bits will give you 1024, you know that 9 is 512 and 11 is 2048. I think 12 bits is the typical maximum I see for in-your-head questions, but if you have to go further, just double away. Hopefully you remember how to carry those ones from grade school

EdTheLad

dynamik wrote: »

Just remember that the number of hosts/networks doubles as you add bits and halves as you subtract bits.

I'm still confused, so if i have 3 hosts and i add a bit i get 6 hosts? add a bit of what?

networker050184

EdTheLad wrote: »

I'm still confused, so if i have 3 hosts and i add a bit i get 6 hosts? add a bit of what?

A bit of miracle grow.

snickered

FluxCapacitor, post a few problems you have trouble with. Perhaps some of us could answer them and you can choose which method makes the most sense to you. Subnetting is incredibly easy once you understand it no matter the class.

LT72884

I use my fingers to subnet. for example on my left hand i do the follwoing.

say you have a /27 bit mask. My thumb = /25, my index finger = /26 and middle finger = /27. now if i want to find out how many networks that is in a class C, i count each of the 3 fingers a s a base 2 number. so thumb = 2, index = 4 and middle = 8 and so forth. Now to find TOTAL IP's i start on thumb and that = 128, index = 64 and middle = 32.

For a /22 same thing. thumb = /23 and index = /22. Then to find how many class C networks are combined is the same step as above. thumb = 2 and index = 4.
to find total ip's the almost the same except rather than going down you double going up. thumb = 512 and index = 1024.

Once you get proficiant at subnetting and super netting you can do stuff like:

summerize the following range

128.3.0.0 - 128.247.0.0

Post some problems and we will help. Ill come up with a chart that might help you.

LT72884

EdTheLad wrote: »

I'm still confused, so if i have 3 hosts and i add a bit i get 6 hosts? add a bit of what?

Say you have a /25 subnet mask which means 25 bits in the subnet mask that are all ones. ?25 means 2 networks with 128 TOTAL ip's and 126 usable. add another bit to the /25 to make it /26 and your hosts will be cut in half and networks will double. /26 = 64 total ips or 62 usable and 4 networks. Once again add another bit to make it /27 and your total IP's get cut in half and your networks double./27 = 32 total IP's and 8 networks.

I read todd's book. My friend is his co writer for it but i did not like it as much as i thought. it does not cover enough for me. I have taken the ccna before and i know what is on the test. His book does not cover as much as i think it should. Just my personal opinion. For example. it does not cover classfull routing table look up vs classless routing table look up for child parent relationships. Something that i have encountered before on the ccna

EdTheLad

LT72884 wrote: »

Say you have a /25 subnet mask which means 25 bits in the subnet mask that are all ones. ?25 means 2 networks with 128 TOTAL ip's and 126 usable. add another bit to the /25 to make it /26 and your hosts will be cut in half and networks will double. /26 = 64 total ips or 62 usable and 4 networks. Once again add another bit to make it /27 and your total IP's get cut in half and your networks double./27 = 32 total IP's and 8 networks.

What happens if i want to add two bits simultaneously rather than only 1 bit at a time?

tim100

FluxCapacitor wrote: »

I am preparing for the ICND1 exam. I am reading the sybex book by Todd Lamle and I was wondering if I should expect to see subnetting questions on class A and B networks because I can subnet class C networks just fine but when I get to class B where the numbers become huge I struggle. Will most of the subnetting questions be on class C? What should I expect to see on the exam as far as class goes?

Thanx

Don't worry. I don't think they are going to ask you to subnet a class A with a /27 mask and list all 524288 subnets.

LT72884

EdTheLad wrote: »

What happens if i want to add two bits simultaneously rather than only 1 bit at a time?

ok so you have a /25 add two bits to make /27 and just remember the rules of base two numbers. if /25 = 128 total ip's then /26 is 64 and /27 is 32. Thats why i use my fingers to count. I always count one bit at a time so i dont get confused. Thats why i use my fingers. it is easier. I wish i could meet you in person so i could help..

Try this chart i made for you see if it helps

the first line with the /25 - /32 == class C ip address masks and the number above it = the total IP's for example. /25 has the number 128 above it for total number of IP's in a /25.. to find networks you need to use some simple algerbra. 128(x)=256 or 256/x=128.. But i will make it simple on you. Its a pattern.

/25 = 2nets 128 ip
/26 = 4nets 64 ip
/27 = 8nets 32 ip
/28 = 16nets 16 p
/29 = 32nets 8 ip
/30 = 64nets 4 ip

notice the pattern above??

Now when you go to the /17 - /23. that shows how many class C NETWORKS you have. for example. the /17 has the 128 above it which means it has 128class C networks in it.

10.1.0.0 - 10.1.128.255 = a /17 mask. to find total IP's you take your networks and times them by 256 because each network has 256 total ip's

the third line is how many class B there are with the mask. So /12 has 16 class B networks. ONE THING TO REMEMBER is that a CLASS B network has 65536 total IP's. To figure that out you take a /32 and subtract you mask from it and then do 2^x where x = product of /32 minus your mask. So take 32 and subtract 16(class b network) and you get 16. 2^16 = 65536.. ok now take a /14 and look at the chart. it has 4 class b networks So you can either find total ip's two ways. first 65536(4) = 262144 OR 32-14 = 18 then 2^18 = 262144. so either way you get the right answer.

PM me if you need more help

thanx

EdTheLad

LT72884 wrote: »

Now when you go to the /17 - /23. that shows how many class C NETWORKS you have. for example. the /17 has the 128 above it which means it has 128class C networks in it.

10.1.0.0 - 10.1.128.255 = a /17 mask. to find total IP's you take your networks and times them by 256 because each network has 256 total ip's

Shouldn't the range be 10.1.0.0 - 10.1.127.255?
What about /16 can this not tell me how many class C networks i have?

Neeko

Why is there talk of how many class C networks a certain mask indicates?

Where is the need to do that and more importantly, where is the logic behind it?

EdTheLad

Neeko wrote: »

Why is there talk of how many class C networks a certain mask indicates?

Where is the need to do that and more importantly, where is the logic behind it?

Not sure how we arrived at it,but its supernetting i.e. CIDR. Used in BGP to keep the routing table as small as possible.Its basically the same concept of aggregating subnets but the mask is to the left of the natural mask.

Neeko

EdTheLad wrote: »

Not sure how we arrived at it,but its supernetting i.e. CIDR. Used in BGP to keep the routing table as small as possible.Its basically the same concept of aggregating subnets but the mask is to the left of the natural mask.

Ah ok, I'm down with that ****.

Let's take 192.168.10.0 /23. By 'how many class C networks', I'm assuming you mean how many are in each supernet? In this case, there are 2 class C networks in each?

EdTheLad

U got it.

LT72884

EdTheLad wrote: »

Shouldn't the range be 10.1.0.0 - 10.1.127.255?
What about /16 can this not tell me how many class C networks i have?

CRAP i FAT fingered it.. yes you are right

SORRY please forgive

Neeko

Well then, a /16 can tell you how many Class C networks you have, 256.

EdTheLad

Neeko wrote: »

Well then, a /16 can tell you how many Class C networks you have, 256.

Exactly

captobvious

Subnetting Made Easy

Check it out!

LT72884

EdTheLad wrote: »

Exactly

heres the kicker. if you know that one network ca support 256 total ip's and there are 256 networks. Just multiply them together to get total IP for /16 256(256) = 65536

Kinda cool to see the patterns

Do you know the rules of supernetting yet?

for example here is a problem we did in class

Write an ACL that allows this range
(i will give you a hint. One mask wont cover that range)

192.168.0.0 - 195.255.255.255

EdTheLad

I don't need supernetting rules, i didn't need your hint either .

access-list 1 deny 192.128.0.0 0.31.0.0
access-list 1 deny 192.160.0.0 0.7.0.0
access-list 1 permit 192.128.0.0 3.127.255.255