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Computer Networking Discuss Networking and Internet connections. Tips on building a LAN or connecting a router.

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Old 04-20-2012   #11
Zero Tolerance
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Subnetting is a little bit confusing to say the least. It took me a while to understand it.

The reason you subnet is to get more usuable IP addresses(or less whichever the case may be). Without being able to subnet, you would be stuck with 255 IP addresses, which leaves you with 254 usable as you cannot use the last one x.x.x.255 (255 is used to broadcast on).

So in order to have more than 254 usable IP's you have to "borrow" some bits to gain a higher number of hosts, or IP addresses.

You can also use subnetting to lower the number of IP addresses available. ISP's do this so you only get ONE usable IP address. This would be a /30 CIDR notation...or 192.168.1.1/30. This would be 255.255.255.252, which would give you 2 usuable hosts (1 would be used for your modem...the other assigned to your router (if you have one) or to the solitary PC if you only have 1 computer. You can minimize the number of hosts by subnetting to a smaller host count for security purposes too. I mean if you have 5 devices on your network...why have 249 free hosts to use left over? IF you are that security concience in the first place that is.

To further explain it...there are 4 octets in a SNM, each with 8 bits. So 255.255.255.0 would be a /24 in CIDR notation. You use binary to count. So a 255 represents a full 8. 254 would represent 7...so forth and so on. You also would count from highest bit position to lowest. So, each bit in binary is represented by counting their position. Again using 7 for example

1 2 4 8 16 32 64 128

You would add 128+64+32+16+8+4+2 = 254

so 8 8 8 7 would = 255.255.255.254

A 255.255.254.0 would be a /23 in CIDR notation. This is the full 8 bits in the first 2 octets, and 7 bits in the 3rd...and 0 in the 4th. 8+8+7+0=23. This allows for 510 usable IP addresses and is the convention I use at work as I dont have but just over 200 devices on my network.

As you can see...subnetting can allow you to customize your host range from something of a very teeny tiny small network...to a huge massive one! And this is just for IPv4. IPv6 allows for even more hosts!

So, as you can see...having a 255.0.0.0 or basically a /8 in CIDR subnet routine...you had MILLIONS too many IP addresses available. Why this would cause you issues...I have no idea right off the top of my head, but if I were to guess it would probably be that the ISP, or better yet your modem/router was on that same subnet and things just got confused.

Again...wasnt trying to sound like a know it all here. Just wanted to try and explain this and clear up the misinformation.
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Old 04-20-2012   #12
Greg
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That is a good read, ZT. Thanks!
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Old 04-20-2012   #13
Zero Tolerance
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No problem. I still like to "geek out" every once and a while. LOL
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Zero Tolerance
aka "SpeedDemon" & "theMadHacker"

As I was walking up the stairs
I met a man who wasnt there
He wasnt there again today
I wish...I wish he'd go away!

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