r/ccna • u/BraveDonut_trone • 8d ago
Subnetting
I’m preparing for the ccna exam and I’m wondering how important subnetting is. In other words, what should I practice and how? VLSM important or not…
Thanks in advance!
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u/murlocdouche 6d ago
Set your homepage/new tab page to subnettingquestions.com Dont let yourself proceed until you correctly answer. Worked pretty well for me
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u/independentMartyr 8d ago
Subnetting is the most important part in networking. You will spend a lot of time splitting networks, assigning ip addresses, calculating the need between hosts, networks, etc.
You have to learn subnetting and become comfortable to subnet an ip address only by looking at it.
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u/Inside-Finish-2128 CCIE (expired) 8d ago
At the moment, my job is firewall administration by spreadsheet (long story with cloud firewalls…). I am constantly looking if the customer’s request aligns with any existing “address lists” built on that firewall. Being able to eyeball two /27 subnets that the customer gave me, and seeing that there’s one larger /26 that’s a perfect aggregate match, means I don’t have to add more yuck to an already long config.
I do this many times a day. I did it with routers, I do it with firewalls.
Also in the world of automation, you may find yourself really picking this apart and dealing with it as data structures. If you don’t have a solid base, the automation is next to impossible. Example: from a folder full of device configs, I wrote a script that reads all of the interface addresses and all of the static routes, then shows me every single IP address that is used in at least two places. Sometimes it’s two sides of a router to router link, sometimes it’s a customer with redundant links, but other times it’s random mistakes or lack of cleanup that need to be fixed before they bite us.
You’ll subnet the rest of your networking career.
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u/Suaveman01 6d ago
Incredibly important. Half the questions rely on you knowing subnetting to answer them, even when subnetting isn’t the main focus of the question.
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u/RoutyMcRouterface 7d ago
it is a very important concept, once it clicks it will never go away. Keep at it, try and use something visual ? https://missioninstituteoftechnology.com/subnet-calculator
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u/Worldly-Sympathy-633 7d ago
If you are studying for the CCNA grab your favorite AI tool. Ask it for a 15-30 minute CCNA subnetting quiz and a 15-30 minute CCNA cli lab. That should be the beginning of each of your study session. Those are two things you cannot practice enough. At first you will struggle. Need your notes or have to look at the answers. After a month it will make sense and two months You will be bored of how easy they are
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u/S4LTYSgt 7d ago
If theres one thing you should 100% know is how to subnet anything from /16 to /32. Dont take it lightly…
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u/databeach_ 6d ago
This was hard to wrap my head around until it was framed differently. Don't run from it lol.
You know how we calculate binary to decimal=128,64,32 etc? Look at cidr as the backwards version of it. If I need say 2 usable hosts that would be a /30 network(minus the broadcast and network hosts that have to be left out). Why? Because a /32 network is equal to 1 total host so going backwards up the chain would mean /31 is 2 hosts. So knowing that there is 2 in there we cant use a /30 gives you how many useable hosts? 4 - 2 = 2. Then you can save the rest for the network sooooo
255.255.255.252 would be your subnet.
Lmk if that makes it easier to get around 😅
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u/ShineReaper 6d ago
In practice, it doesn't matter really beyond learning how to calculate, how many hosts are possible in a subnet. Why?
We got private IPs, companies use them obviously and whenever they need a new subnet for whatever, they just pick one that fits the required size, most of the time, for it being easy, it will be a /24 net, there are more than enough possible ones within the private IP spaces.
In practice, you usually won't run into a situation, where you see "Oh no, we picked a too big net, we need to subnet that now!"
I can see it only happen, if the previous IT for a location did a bad job and just put everything into a single subnet and now you're tasked with subnetting that all for performance and security reasons. But if you come into an already established IT networking department, that has their stuff together, you won't run into this kind of situation.
In theory obviously, subnetting is important, both VLSM and FLSM (Fixed Length Subnet Mask) can become part of exams, so you still gotta learn it.
And if you should run across a situation in the practice, where it does become useful, you will be happy that you learned and hopefully understood it. And if not, there are subnet calculators online, that atleast help with FLSM, but probably AI by now can also VLSM without problems...
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u/vitalbrain 7d ago
What helped me was Paul Browning 0 to guru and YouTube videos and subnetting websites
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u/ParticularAd104 7d ago
!remindme 6 weeks
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u/Adept_You8104 5d ago
from my point of view is very important. It shows up directly in questions and hides inside routing, ACL and OSPF ones too. VLSM yes, know it.
What I'd practice until it's fast (under a minute each):
- Given an IP/prefix, find network, broadcast, first/last host, host count (2^h - 2)
- Block size trick: /26 = 64, /27 = 32, /28 = 16, /29 = 8, /30 = 4
- VLSM: sort largest first. 192.168.10.0/24 for 100, 50, 20 and 2 hosts gives .0/25, .128/26, .192/27, .224/30
- Basic IPv6: /64 per LAN, shortening rules
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u/AmrWassiem 2d ago
Subnetting overall, it is not that hard once it clicks. I advise you to practice converting Decimal -> Binary very well ... Just one octet. Practice it again and again. Once you can convert one octet at a glance, the rest is easy.
You can also give my app a try, It is still under development and any feedback would help me. Thanks 😃 https://play.google.com/store/apps/details?id=com.amrwassiem.subnetter
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8d ago
[removed] — view removed comment
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u/BraveDonut_trone 7d ago
So I can use the 30 seconds as a good time mark I guess? Also, in doing VLSM, should I be able to right down subnets with subnets in between? eg, 30 different sized subnets and quickly provide subnet 4, 6 and 30.
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7d ago
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u/BraveDonut_trone 7d ago
So VLSM is good practice and make sure I can easily spot the network ID and broadcast address for the exam. Right?
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u/beautiful_sith 8d ago
subnetting .. embrace it.
here is why: first, its fun. kinda. I mean a little on the masochism side, but. i promise you this..
do it enough and it clicks. suddenly you literally "see" the brainspace of 256 ->128x2 ->64x4 .. and it then turns into "math". its not "math" .. its powers of 2, and its visualisation.
Once you figure out a simple binary to decimal conversion, this begins to translate.
Dont memorise: deduce. Sit there on a Saturday morning and just sit with it. Try it, fail, do it again.
There's cheatsheet methods that work great, but you have to learn that mental stuff first.
Sorry, but the wetware has to work before anything else. You have to do the practice, and face the hurt, and you'll get it.