CIDR and /number???

[berderder]

Can someone give me a simple explanation of CIDR (classless interdomain routing) and what the /*number* after an IP address means? I've been having a hard time wrapping my head around it.

 

Thank you.

[Lurick]

10.200.1.0/25

The subnet would be:

10.200.1.0 - 10.200.1.127

1.0 is the subnet

1.1 is the gateway

1.127 is the broadcast

255.255.255.128 is the subnet mask, which in this case is /25

 

Another example:

 

10.10.0.0/20

 

the /X is the subnet for the network.

 

 

----Edit----

To explain it better there is classful and classless routing. Classful routing is the old school and original way of doing things where you have your Class A, Class B and Class C subnets.

Class A subnets range from: 0.0.0.0 to 127.x.x.x with a subnet mask of 8 which is 255.0.0.0

Class B subnets range from 128.0.0.0 to 191.x.x.x with a subnet mask of 16 or 255.255.0.0

Class C subnets range from 192.0.0.0 to 223.x.x.x with a subnet mask of 24 or 255.255.255.0

 

You will still see this today in Windows, which baffles me but that's besides to point, where you'll type in say 192.168.1.x and it will fill out the subnet field with 255.255.255.0 or if you put 10.1.1.0 it will fill out the subnet field with 255.0.0.0, it assumes classful networks.

 

Classless routing is where you can take any network and subnet it however you damn well please. You can have for example 10.0.1.0/24 or 192.0.0.0/8

 

Routing protocols originally only used to be able to pass classful subnets so even if you had 10.0.1.0/24 it would pass it to the neighbor router as 10.0.0.0/8 which made things every difficult.

[KuJoe]

Disclaimer: I am not the best at networking so I'll describe it in a way that makes sense to me.

 

An IPv4 address is 32 bits total (when converted to binary). The /# is the number of bits for the routing/networking and the remaining bits are for the smaller network (subnetwork). So if you have a /0 then it covers all IPv4 addresses available, if you have a /32 it covers on a single IP address, if you have a /31 it covers 2 IPs, /30 is 4 IPs, and so on.

[berderder]

6 hours ago, Lurick said:

10.200.1.0/25

The subnet would be:

10.200.1.0 - 10.200.1.127

1.0 is the subnet

1.1 is the gateway

1.127 is the broadcast

255.255.255.128 is the subnet mask, which in this case is /25

 

Another example:

 

10.10.0.0/20

 

the /X is the subnet for the network.

 

 

----Edit----

To explain it better there is classful and classless routing. Classful routing is the old school and original way of doing things where you have your Class A, Class B and Class C subnets.

Class A subnets range from: 0.0.0.0 to 127.x.x.x with a subnet mask of 8 which is 255.0.0.0

Class B subnets range from 128.0.0.0 to 191.x.x.x with a subnet mask of 16 or 255.255.0.0

Class C subnets range from 192.0.0.0 to 223.x.x.x with a subnet mask of 24 or 255.255.255.0

 

You will still see this today in Windows, which baffles me but that's besides to point, where you'll type in say 192.168.1.x and it will fill out the subnet field with 255.255.255.0 or if you put 10.1.1.0 it will fill out the subnet field with 255.0.0.0, it assumes classful networks.

 

Classless routing is where you can take any network and subnet it however you damn well please. You can have for example 10.0.1.0/24 or 192.0.0.0/8

 

Routing protocols originally only used to be able to pass classful subnets so even if you had 10.0.1.0/24 it would pass it to the neighbor router as 10.0.0.0/8 which made things every difficult.

 

6 hours ago, KuJoe said:

Disclaimer: I am not the best at networking so I'll describe it in a way that makes sense to me.

 

An IPv4 address is 32 bits total (when converted to binary). The /# is the number of bits for the routing/networking and the remaining bits are for the smaller network (subnetwork). So if you have a /0 then it covers all IPv4 addresses available, if you have a /32 it covers on a single IP address, if you have a /31 it covers 2 IPs, /30 is 4 IPs, and so on.

I have a textbook for the CCNA certification. I feel like that would be really helpful. I've read a chapter or two in.  Right now I am studying for my CompTIA A+ and this seems like a larger than usual networking topic that they hit on potentially on the exam. 

[.:MARK:.]

It's a very essential part of any networking syllabus.

/number is just the number of bits in the subnet mask, so /24 is 24 out of 32 bits available. 255.255.255.0

[Holen]

Mark what you wrote is the wrong way around.

If the CIDR code is 24, the the first 24 bits in the subnetmask are set.
This means that the first three pairs of the IP corresponds to a spesific subnet.
If you have an ip of 192.168.0.0/24, then any widget with an ip between 192.168.0.0 and 192.168.0.255 are on the same subnet.
Which in turn means there are 32-24=8 available bits for host addresses, though the first and last ip are reserved for the gateway and broadcast addresses.

[berderder]

2 hours ago, Holen said:

Mark what you wrote is the wrong way around.

If the CIDR code is 24, the the first 24 bits in the subnetmask are set.
This means that the first three pairs of the IP corresponds to a spesific subnet.
If you have an ip of 192.168.0.0/24, then any widget with an ip between 192.168.0.0 and 192.168.0.255 are on the same subnet.
Which in turn means there are 32-24=8 available bits for host addresses, though the first and last ip are reserved for the gateway and broadcast addresses.

To me it's sort of a complicated networking concept that I am surprised appears on the CompTIA A+ certification at all. CCNA would make sense but it seems a bit elevated for CompTIA A+

[Lurick]

33 minutes ago, berderder said:

To me it's sort of a complicated networking concept that I am surprised appears on the CompTIA A+ certification at all. CCNA would make sense but it seems a bit elevated for CompTIA A+

It's absolute basic networking, being able to subnet a given address and tell how many host addresses you'll have what the gateway and broadcast address is. There is a reason they spend a good 1/3 of the CCNA books talking about it and pounding it into your brain. Just wait until you get into routing protocols like OSPF or even OSPFv3, that's where the real fun begins

[berderder]

3 minutes ago, Lurick said:

It's absolute basic networking, being able to subnet a given address and tell how many host addresses you'll have what the gateway and broadcast address is. There is a reason they spend a good 1/3 of the CCNA books talking about it and pounding it into your brain. Just wait until you get into routing protocols like OSPF or even OSPFv3, that's where the real fun begins

Well, I can't argue with that

[Lurick]

Just now, berderder said:

Well, I can't argue with that

It takes everyone a while to get subnetting, it doesn't come easy but it's well worth it in the end and you'll usually reach that point after a few weeks of studying it and playing around where it will just click. And then some jerk comes along with IPv6 and tells you to subnet that

[Holen]

Yes, there is nothing more satisfying that having 4 months to read up on 10 months worth of stuff.
Having to be able to distinguish between OSPF and it's priority system versus other routing protocols.

As well as having to do this while working part time, and having subjects such as hacking and scripting...

To anyone wondering, the CCNA exam is considered the most difficult beginners certification. if you take the CCENT then the CCNA, you will realize the CCENT is a piece of cake compared(as it covers basically nothing)

[berderder]

19 hours ago, Holen said:

Yes, there is nothing more satisfying that having 4 months to read up on 10 months worth of stuff.
Having to be able to distinguish between OSPF and it's priority system versus other routing protocols.

As well as having to do this while working part time, and having subjects such as hacking and scripting...

To anyone wondering, the CCNA exam is considered the most difficult beginners certification. if you take the CCENT then the CCNA, you will realize the CCENT is a piece of cake compared(as it covers basically nothing)

CCENT sounds like a nice little cert to get. After I get my CompTIA A+ I will probably work on that. I am more interested in hardware than networking, but since some networking is probably inevitable, a CompTIA A+ with CCENT seems like a nice duo.

[Holen]

41 minutes ago, berderder said:

CCENT sounds like a nice little cert to get. After I get my CompTIA A+ I will probably work on that. I am more interested in hardware than networking, but since some networking is probably inevitable, a CompTIA A+ with CCENT seems like a nice duo.

If you go for a CCENT, you should atleast consider a CCNA. It is difficult, but it is worth it for most people

[berderder]

15 minutes ago, Holen said:

If you go for a CCENT, you should atleast consider a CCNA. It is difficult, but it is worth it for most people

Yes, my goal would be to turn that CCENT into a CCNA next

[Lurick]

9 minutes ago, berderder said:

Yes, my goal would be to turn that CCENT into a CCNA next

And then go for the three CCNP exams

[Holen]

1 hour ago, Lurick said:

And then go for the three CCNP exams

CCNE after?  when you've worked in the business for a long enough while

[BSpendlove]

On 23/05/2016 at 6:45 PM, Holen said:

Which in turn means there are 32-24=8 available bits for host addresses, though the first and last ip are reserved for the gateway and broadcast addresses.

Sorry to quote but I just wanted to clarify and make sure I don't confuse myself... You say gateway but isn't the first address (eg. .0) is to identify the network / Network ID?

[Lurick]

1 hour ago, BSpendlove said:

Sorry to quote but I just wanted to clarify and make sure I don't confuse myself... You say gateway but isn't the first address (eg. .0) is to identify the network / Network ID?

Yah, the first IP (eg. 0) is used to ID the network. I think he was referring to the 1 in lets say a /24 which is typically the gateway, although doesn't have to be.

[leadeater]

1 hour ago, BSpendlove said:

Sorry to quote but I just wanted to clarify and make sure I don't confuse myself... You say gateway but isn't the first address (eg. .0) is to identify the network / Network ID?

 

Edit: Nvm reading a bit more of the thread makes your question make a bit more sense in context.

 

 

No .0 it is not always the network address and .255 isn't always the broadcast address. Most home networks use /8, /16 or /24 masks so this is true for these and is so common it is understandable to think that it is always the case.

 

If we take an IP of 172.16.1.0/16 as an example and convert it down to bit octets you can calculate the network address and broadcast address.

 

Host:   172.16.0.1                     10101100.00010000.00000000.00000001
Subnet mask:  255.255.0.0       11111111.11111111.00000000.00000000

Network:  172.16.0.0                 10101100.00010000.00000000.00000000

Broadcast: 172.16.255.255       10101100.00010000.11111111.11111111

 

But now if you take 172.16.0.1/17

 

Host:   172.16.0.1                      10101100.00010000.00000000.00000001
Subnet mask:  255.255.128.0   11111111.11111111.10000000.00000000

Network:  172.16.0.0                 10101100.00010000.00000000.00000000

Broadcast: 172.16.127.255       10101100.00010000.01111111.11111111

 

And now 172.16.128.1/17

 

Host:   172.16.128.1                  10101100.00010000.10000000.00000001
Subnet mask:  255.255.128.0   11111111.11111111.10000000.00000000

Network:  172.16.128.0             10101100.00010000.10000000.00000000

Broadcast: 172.16.255.255       10101100.00010000.11111111.11111111

 

You may also find this thread interesting as it covers more about subnet masks, yea I recycled one of my posts in this one for here if your wondering why I used 172.16 addresses. I also used host addresses on the boundaries of subnets on purpose, give 192.168.72.27/19 a try and see what you get for network address and broadcast.

 

Edited May 27, 2016 by leadeater

[leadeater]

9 minutes ago, Lurick said:

Yah, the first IP (eg. 0) is used to ID the network. I think he was referring to the 1 in lets say a /24 which is typically the gateway, although doesn't have to be.

Yea normally I use the last IP in a subnet as the gateway rather than the first, just something I like to do mostly.

[BSpendlove]

17 minutes ago, Lurick said:

Yah, the first IP (eg. 0) is used to ID the network. I think he was referring to the 1 in lets say a /24 which is typically the gateway, although doesn't have to be.

xD, I have developed the same habit since my boss normally uses 254 on the standard /24