Network Class Table - This is optional. It is very easy to memorize this.
A - 1 - 127 (some books do not include 127)
B - 128 - 191
C - 192 - 223
Subnet Mask Table
/9,/17,/25 - 128
/10,/18,/26 - 192
/11,/19,/27 - 224
/12,/20,/28 - 240
/13,/21,/29 - 248
/14,/22,/30 - 252
/15,/23,/31 - 254
Binary to Decimal Table
4096 2048 1024 512 256 128 64 32 16 8 4 2 1
You can add more numbers to the Binary to Decimal table if you want. Write this down before starting your exam.
So what are those tables above? These tables will help you on how to subnet.
Finding total number of subnets and/or usable subnets.
Find out what the default subnet mask for given IP/Network address. For example, 191.10.10.0/26. This is Class B so it will be /16. It'll look like this: nnnnnnnn.nnnnnnnn.ssssssss.sshhhhhh
Legend:
n = network
s = subnet
h = host
Yes, I broke it down to binary but you're not really going to use it to solve the problem.
How many "s" do you see? 10. This may take time to write down while taking the test right? So I'll give you the easier way to do this, g - d, /26 - /16 = 10.
Legend:
g = Given subnet mask
d = Default subnet mask
So what do you need the 10 for? Well, that's what you need to match up to the Binary to Decimal table. What is the 10th number from right to left? 512 right? What now? Is the answer 512 - 2 subnets? No, look at the number to the left of it, it's 1024 -2 = 1022. So the answer is 1022 usable subnets. Some people would add 512 + 256 + 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1, but the answer is 1023. Where's the other number? You actually need to count the 0 to make it 1024. If you add all those numbers, you're gonna waste a lot of time - 90 minutes is not a lot of time during exam. Knowing the technique that I just mentioned will definitely speed things up.
Cisco questions can be tricky, so make sure that it is asking you for the usable subnets or total number of subnets. If the question asks usable, then always deduct 2. Otherwise, do not deduct 2.
Finding total number of hosts and/or usable hosts per subnet.
Using the same example above, how many "h" do we have, 6 right? What are we gonna do with that number? Let's use the same technique that I showed above. What is the 6th number (right to left) in Binary to Decimal table? 32 right? That's not the answer though. If you recall, the next step is to look at the left of that number, which is 64. So the answer would be, 64 (total number of hosts) - 2 = 62 (usable hosts). Take note of the 64 because this is going to be the range.
There's another way of doing this. What is the given subnet mask? /26 right? What is the equivalent if you have all 1s as the subnet mask? /32 right? 32 - 26 = 6, this is the same number as the above. This is way faster, right?
Finding out total number of hosts.
This is easy now since we've done the total number of subnets and hosts. All you have to do is get the total number of subnets * total number of hosts, 1024 * 64 = 65,536.
How to find usable address, broadcast address, or network address?
We're going to use the same example above, but this time the last octet won't be 0. Let's use 191.10.253.198/26. Let's say the question asked you to pick if this is a broadcast address, network address, or usable address.
This is where range comes in. Earlier, I asked you to take note of that number. If you forgot the number, it is 64. How are we going to use this? Well, 64 is the amount of hosts per subnet. Let's take a look on how it's gonna look like for each subnet.
Subnet 0 - 191.10.0.0 - 191.10.0.63
Subnet 1 - 191.10.0.64 - 191.10.0.127
Subnet 2 - 191.10.0.128 - 191.10.0.191
Subnet 3 - 191.10.0.192 - 191.10.0.255
Subnet 4 - 191.10.1.0 - 191.10.1.63
Subnet 5 - 191.10.1.64 - 191.10.1.127
Subnet 6 - 191.10.1.128 - 191.10.1.191
Subnet 7 - 191.10.1.192 - 191.10.1.255
Do you see the pattern? There are 64 IP addresses in each subnet. This is why I wanted you to remember the range and/or total number of hosts per subnet. We're not quite done yet. We still need to find out the answer. Since there's a pattern, we'll just skip all the subnets except the ones that are close to the given.
Subnet 1015 - 191.10.253.0 - 191.10.253.63
Subnet 1016 - 191.10.253.64 - 191.10.253.127
Subnet 1017 - 191.10.253.128 - 191.10.253.191
Subnet 1018 - 191.10.253.192 - 191.10.253.255 ---- This is where the given address sits and is considered as usable IP address, since the first address of the subnet is the network address and the last address is the broadcast address.
I hope that you learned something from this. Please feel free to comment and/or ask questions.
Written By: Andr01d
"I know nothing except the fact of my ignorance" - Socrates
0 comments:
Post a Comment