It is 2:24 AM on a Tuesday morning and I'm here at work not doing much. Yea I am slacking off a little... So what comes to mind at this time? The new lab I just bought!!! Yes, I sold my old one and bought a better one. I now have 4 2811 Cisco routers, and 3 3750 switches and I also upgraded the modules to 16FE interfaces so I can do some heavy bundling between the routers and switches. This will be my CCIE lab and I am planning to maybe get 1 more switch but that will be later. I am also considering getting a full blown Juniper lab as well, so soon I am going to have 2 decent lab kits from 2 different vendors to get them to work together. Juniper and Cisco, to me, provide the best equipment and certifications in this market and both have a very high demand. With that being said, I wanted to talk about my CCNA experience. I honestly didn't know much about networking. I had some knowledge but definitely not on a CCNA or CCNP level. The real reason that interested me was because it was available for self studies and it was cheap. I never thought the potential that it had until I really got involved. I had bought a decent lab 4 2600 routers and 4 2950 switches. But do you really need 8 devices you might ask? The answer is no. You can definitely have a decent lab by just having 2 routers and 2 switches both the 2600 and 2950 models if money is tight. A website that is good to buy CAT5 cables straight and crossovers is in Monoprice.com. They have very cheap cabling if you dont have the tools to make them. So the equipment was in an ok condition when I bought them but after a while they started failing, which to me was great. I learned to replaced the fans, modules, RAM memory, Flash card, and even the serial cables and interfaces which I had to change the clock rates to get it to work. I also learned how to set it up with my cable modem as the gateway. Everyday I studied for a few hours and on the weekend it was time to hit the lab. The downfall was how loud those routers are and how much power it eats up so beware! I also used Microsoft One Note to take notes. You can take that free app anywhere including your phone and have nice and clean organized notes. You MUST take A LOT of notes when studying this, trust me it can be hard to remember all these protocols. Anyways I wanted to share a subnetting trick that I had posted on my IG account. Once you master that chart, subnetting will come in easy and that will be the least of your worries while taking the test. Remember time will NOT be in your favor while taking the test so you have to really check your time and not spend so much time in one question. I actually failed my first test, the ICND1. I was soooo mad and I hated Cisco for it, but after my second attempt, I passed it fine. So here it is and hopefully you enjoy it....

ATTACKING A SUBNET

Many students nowadays have questions and difficulty understanding what subnetting is. I have made an easy explanation on how to solve subnetting questions to ace the test and to use it in the real world. I will be honest, more than likely you will not subnet a network by memory when you get hired for a networking job. The entire network is already designed. Calculators and programs have been made to ease the pain. But, the purpose of this tactic is for you to attack it on the day you won’t have a calculator in hand, like your test day. Hey maybe you forgot how to subnet because you have been so used to using a calculator like me. Before you continue, you must have some understanding of what subnet masks are and their purposes. If you don’t, then you must go back and hit the books.

First thing is first, having a visual chart will help you tremendously when subnetting a network. So let’s first start with this chart. When the day comes that you have to take your CCNA test, I would HIGHLY recommend writing out this simple chart quickly before the test starts. The test does not have enough time for you to spare.

128 64 32 16 8 4 2 1                    

192

224

240

248

252

254

255

So what do you do when you have 192.168.1.0 /25 mask? Well you must know that a subnet mask is composed by 8 bits for every decimal.

255.255.255.255 = 8bits.8bits.8bits.8bits = 32bits

So what happens when you have a /25 mask? You just add the first 3 decimals equal to 24 bits. Then you only have 1 left right?

255.255.255.? = 255.255.255. (Add the first one from the graph horizontally which is 128)

255.255.255.128 is your subnet mask

 

 

 

 

What about a /26 mask? 255.255.255.? = 255.255.255. (Add 2 from the graph horizontally which is 128 + 64 = 192)

128 64 32 16 8 4 2 1                    

192

224

240

248

252

254

255

One more example, what about a /27 mask? = 255.255.255. (Add 3 from the graph horizontally 128+64+32= 224)

128 64 32 16 8 4 2 1                    

192

224

240

248

252

254

255

As you can see, your answers are already in the graph. You just need to add the same amount of bits that you used horizontally to vertically. Now, what do you do when you have the subnet mask? You subtract 256 from whatever you have left on the subnet mask. Using a 192.168.1.0 /25 mask, you subtract 256 from 128 and that leaves you with a 128 subnet block. This means that you only have 2 subnets to use. Remember to subtract 2 for the subnet ip address and broadcast ip address. EASY RIGHT?!!!!

256-128 = 128 subnet block

192.168.1.0 ß

192.168.1.128

192.168.1.256

 

SUBNET IP                MASK

192.168.1.0             255.255.255.128

USABLE IP ADDRESSES                           BROADCAST IP

192.168.1.1 to 192.168.1.126                192.168.1.127          

After attacking this /25 subnet, now you know that you can have 2 subnets and 126 hosts in your network.      

Ok one more that is not a common one. What is the subnet mask and usable IP addresses for 15.25.13.16 /28? =

We already know the first 3 decimals are 24 bits. Add 4 bits to make it 28.

128 64 32 16 8 4 2 1                    

192

224

240

248

252

254

255     

256-240= 16 subnet block

15.25.13.0

15.25.13.16  ß

15.25.13.32

 

Subnet IP 15.25.13.16                            Mask 255.255.255.240

Usable IP                                                   Broadcast IP

15.25.13.17 to 15.25.13.30                   15.25.13.31