IP Subnet Planning, I need your help

[mikesx4911]

I have tried and tried to try to figure this out and I am not getting anywhere. Can someone please explain this formula to me? I'm using Packet Tracer trying to do this lab which I have attached and can't get past the planning of the network. It tell's me that you start with a IP of

172.16.0.0 /22

where do I go from there if I need 7 subnetworks with a different number of hosts on each one?

By no means am I asking you to do my homework for me or to just give me the answers, I want to learn this skill. I have used CBT nuggets video to try to practice which I understand their concept but I'm confused on this can even apply to this. I have looked up and found some of the answers to this btw but I can't figure out how they are coming up with these subnet masks. Any help is appreciated. HAPPY EASTER!!!

PacketTracer533_setup_no_tutorials.exe
(packet tracer application)

LSG01-PTSkills7.pka
(packet tracer lab)

 

[Lexluethar]

Well the way i remember is /24 notation will give you 255.255.255.0. Which is one subnet and 254 hosts (256 - 2, you subtract two). So a /24 gives you 254 hosts and is 1 subnet. From there for the hosts you just double every time you go down, and subtract half when you go up. Meaning if /24 is 256 hosts (i know, its actually 254 but if we always subtracted two you'd end up taking too many out with each notation), /25 is 128 hosts, /26 is 64 hosts, /27 is 32 hosts, etc.

Now on the flip side of that if you go down to /23 you double it, so /23 is 512, /22 is 1024, etc. Just keep doubling it. As for the subnets that are created it depends, you start out with 1 subnet with a /8, /16, /24 subnet. Why? because you aren't borrowing bits to create multiple subnets
/8 is 255.0.0.0 which is one subnet
/16 is 255.255.0.0 which is one subnet
/24 is 255.255.255.0 which is one subnet

Now for subnets you just double every time you go up from the numbers above. IE /16 is one subnet, /17 is 2, /18 is 4, /19 is 8 ect.

So the two rules really to remember that were the easiest for me are as follows:

1. When dealing with hosts always start is the /24 notation. So you always start out with 256 hosts. From there you can do the math (remember if the number is larger take half, if the number is smaller double) from there and subtract two once you get to the number you need.
2. For subnets remember those three important subnet numbers i gave you, any number above those you double each time (IE /8 is 1 subnet, /9 is 2, /10 is 4, /11 is 8, etc)

I'm not going to give you the answer, but those those two rules you should be able to figure out any subnet problem you encouter. For your example you need to determine how many hosts you need for each subnet, then just find the subnet that fits your need for hosts (per subnet) and 7 subnets.

There is a great chart out there that gives you every possible subnet, the subnet range and the notation with each number. I'm not going to put that up here because it would probably give you the answer. If you PM me i'll give it to you in a few days.

 

[mikesx4911]

awesome reply that make's sense, probably the best way I have heard it explained so far. I broke out a paper and pen and tried to calculate the subnet mask for the biggest subnet which was 400 hosts. So I remember you said with one subnet can handle 256 address -2 one for broadcast and one for network, so I came up with that I should be using 255.255.254.0 for 400 hosts which would give me 508 more than enough. I checked and I am correct on that one so yipeee, it's finally making some sense haha. So I tried to do the 2nd largest subnetwork which was 180 address and I instantly thought so myself well one subnet can handle 180 easily so it should be 255.255.255.0 I checked and that's not the right answer, but I'm confused as to why it's not. It had more than enough addresses. Am I missing something?

 

[Lexluethar]

Are there any other requirements like amount of subnets? You are right /24 gives you 254 hosts, the next one down which is 255.255.254.0 is technically 510 hosts and 255.255.255.128 is 126 hosts.

I don't think you are missing anything, is there something else in the equation we should now?

 

[mikesx4911]

The subnet requirements are with the ip they gave me of 172.16.0.0 /22

Subnet assignments are:
1st subnet, existing student LAN, up to 400 hosts; (Fa0/0 on R2-Central)
2nd subnet, future student LAN, up to 180 hosts; (not yet implemented)
3rd subnet, existing ISP LAN, up to 40 hosts; (Fa0/0 on R1-ISP)
4th subnet, future ISP LAN, up to 18 hosts; (not yet implemented)
5th subnet, existing WAN, point-to-point link; (S0/0/0 on R1-ISP and R2-Central)
6th subnet, future WAN, point-to-point link; (not yet implemented)
7th subnet, future WAN, point-to-point link. (not yet implemented)

So I got the biggest one of 400 users and I'm trying to get the rest but they keep coming up wrong. One subnet can handle more than 180 users so I would of thought it would just be 255.255.255.0 for the next biggest subnet of 180 users but they say it is wrong, this is why I think I'm missing something.

 

[mikesx4911]

I also need to know how to find out what the ip range is from a subnet mask. My ultimate goal is to find out what the ip address should be for

R1-ISP's S0/0/0 interface, configure the highest usable address on the existing WAN subnet. which I know the sub is 255.255.255.252

 

[office politics]

i usually use the site linked below when trying to calc numbers for subnets.

Online IP Subnet Calculator

 

[mikesx4911]

Thanks everyone, I know understand the whole idea behind subnetting. Lex you should be a instructor that was the simplest set of instructions I have heard yet on subnetting. I was able to do my homework

 

[Lexluethar]

Glad you figured it out. I'm happy to help, i just recently felt comfortable with subnetting myself. Those two rules honestly are the key, you can pretty much figure out everything from there as long as you remember those.

So what was the final answer with 180 hosts?

 

[mikesx4911]

Glad you figured it out. I'm happy to help, i just recently felt comfortable with subnetting myself. Those two rules honestly are the key, you can pretty much figure out everything from there as long as you remember those.

So what was the final answer with 180 hosts?

Needing 180 hosts I would only need one subnet of 255.255.255/24 which gives me 256 - 2 for the broadcast and network. This give's me an range of
172.16.2.1 - 172.16.2.254

Fairly easy once you get the hang of doing it.