Subnetting on the fly...

[DreadStarX]

Hey Guys!


It's been quite some time since I've posted here! I've been quite busy, and finally received my first certification ever, for PCs!

I'm A+ Certified now!

But to the point!

I've posted on a few other tech support sites I visit, and help out on occasionally, but nothing too serious has been posted.

I'm currently enrolled in a Cisco CCENT course at my school. I'm having major problems with subnetting and learning it.

If your IP is 192.168.1.1 / 64.24.96.103 (Public IP), how do you know that you're subnet is going to be 255.255.255.0? How can you tell at a glance that it's going to be that way.

I'm struggling really badly with this.


- Thomas

 

[TanyaC]

Hi,

Re your private addresses (192.168.1.x): This is a class C address range (Private network) that is not visible on the internet.

A subnet of 255.255.255.0 means you can have 256 addresses (but you can't really use 0 and 255). You can actually use a different subnet if you want (assuming you needed that many addresses. For example, 255.255.0.0 would give you 256 x 256 (65536) addresses, all for your own personal use (again, as they can't be used on the public Internet).

If you wanted to limit the number of addresses you might use 255.255.255.192 which would split your network into 4 subnets. This can get complex, so you might want to look at Understanding TCP/IP addressing and subnetting basics or IPv4 - Wikipedia, the free encyclopedia if you really want to understand IPv4.

Regarding you public address:

Typically an ISP will use a subnet of 255.255.255.255. This means that you get only one address, the public address they give you. This is how they segment and allocate their finite IP address range. It also help with security from their perspective, as it helps prevent spoofing.

 

[DreadStarX]

Hi,

Re your private addresses (192.168.1.x): This is a class C address range (Private network) that is not visible on the internet.

A subnet of 255.255.255.0 means you can have 256 addresses (but you can't really use 0 and 255). You can actually use a different subnet if you want (assuming you needed that many addresses. For example, 255.255.0.0 would give you 256 x 256 (65536) addresses, all for your own personal use (again, as they can't be used on the public Internet).

If you wanted to limit the number of addresses you might use 255.255.255.192 which would split your network into 4 subnets. This can get complex, so you might want to look at Understanding TCP/IP addressing and subnetting basics or IPv4 - Wikipedia, the free encyclopedia if you really want to understand IPv4.

Regarding you public address:

Typically an ISP will use a subnet of 255.255.255.255. This means that you get only one address, the public address they give you. This is how they segment and allocate their finite IP address range. It also help with security from their perspective, as it helps prevent spoofing.

The part I'm having issues with is the bits and values

7 6 5 4 3 2 1 0
128 64 32 16 8 4 2 1

I know that each octet has 8 bits. If its 255.255.255.0 its, 8bits, 8 bits, 8 bits, 0 bits. After that I'm lost. I know its

xxx.xxx.xxx.xxx/24 if you're subnet is 255.255.255.0 (Like my home address is). I know about the And rule as well.


1 AND 1 = 1
1 AND 0 = 0
0 AND 1 = 0
0 AND 0 = 0

So much damn information to learn.

 

[Phone Man]

Been a while since I worked with IP but hear are a few thinks I remember. To determine the default subnet mask here is the rules.

• Class A networks use a default subnet mask of 255.0.0.0 and have 0-127 as their first octet. The address 10.52.36.11 is a class A address. Its first octet is 10, which is between 1 and 126, inclusive.
• Class B networks use a default subnet mask of 255.255.0.0 and have 128-191 as their first octet. The address 172.16.52.63 is a class B address. Its first octet is 172, which is between 128 and 191, inclusive.
• Class C networks use a default subnet mask of 255.255.255.0 and have 192-223 as their first octet. The address 192.168.123.132 is a class C address. Its first octet is 192, which is between 192 and 223, inclusive.

So in your example your LAN address starts with 192 and is a Class C with a default mask of 255.255.255.0 and you can use 254 host addresses for your devices.
You Public IP starts with 64 and is a Class A with a default mask of 255.0.0.0 so there are something like 16 million address available on that network. Now is where sub net comes in. By changing the mask you can divide a large network into sub networks. My ISP has a Class A address of 68 but on the network feeding my area they use a mask 255.255.255.0 so there are only 254 addresses available to anyone on this network. When a company buys a block of IP addresses they select what class they want and then are assigned a block of addresses in that class. My ISP owns every address that starts with 68 as it is a class A but uses masks to split them up into multiple networks feeding their customers.


Hope this helps.


Jim

 

[TanyaC]

Hi,

Sorry, I must have misunderstood... Your question was..

If your IP is 192.168.1.1 / 64.24.96.103 (Public IP), how do you know that you're subnet is going to be 255.255.255.0? How can you tell at a glance that it's going to be that way.

Subnet masks are not the "question" they are the answer, so to speak...

If I have a need for a network with 62 hosts, what subnet would I use? (rhetorical)

Regarding the use of binary. In the example you showed, with all bits on (1) the value would be 255. You get this value by adding together all the values of each but. But I guess that much you understand.

Perhaps this subnet calculator will help you a little... Online IP Subnet Calculator

Check out this page, this is where the penny dropped for me.. The first bit is a little bit of waffle (no pun intended), but once you get to the part about "You're losing me..." it concentrates on the bit structure.

Understand Your Subnet Mask

hth

Thanys