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Highlighted
Enthusiast

192.168.1.0/21 ???!!

Help !!

I just came across a network for one of my customers and noticed strange private addressing allocation. They use 192.168.1.0/21 as their subnet and everything work.

I don't understand. How can someone use a class C address using 255.255.248.0 as a subnet mask? .. Class C should be subnetted in a forward direction not backward. For example, host bits are borrowed from the right side (forward) for subnetting, /25, /26, /27,..etc.

How can this affect summarization on routers? What are the pros and cons?

Kindly i need a quick response on this one as i am doing an ongoing network survey for the customer.

Thanks

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12 REPLIES 12
Highlighted
Engager

Have you tried to work out

Have you tried to work out what the network address is?

Secondly, Class C's can be changed to class B's, doesnt necessarily mean it has to go smaller, it can go bigger, just depends on the networking history of their environment, there might have been a use case for it - a perfectly good reason like growth, and expansion.

Please rate useful posts & remember to mark any solved questions as answered. Thank you.
Highlighted
Enthusiast

Yes, the main reason was

Yes, the main reason was increasing the number of hosts but i don't want to discuss their reason now. I want to know (for myself) how does a class C address use a mask that class B owns? ... In subnetting, it is obvious to borrow bits from the host portion to create new subnets. In the case here, a class C address is using a mask that enables us to play in the third octet. Third octet is always the game that class B addresses play.  How does a class C address use the third octet ? ... The only thing i saw that uses class C address with /21 mask is, Summarization. 

By the way, my understanding in subnetting is good, but maybe there are things in subnetting that i still don't know.

 

Thanks

Highlighted
Beginner

Classful routing, which is

Classful routing, which is composed of Classes A, B, C, D, and E, is no longer used.  All routing is classless, but classes are referred to, but have no practical usage today.  The subnet can be increased or decreased according to the number of hosts and subnets needed; however, too many hosts on a subnet can cause problems due to too many broadcast messages being sent.

Highlighted
Beginner

Classful routing, which is

Dup.

Highlighted
Enthusiast

Yes, the main reason was

Yes, the main reason was increasing the number of hosts but i don't want to discuss their reason now. I want to know (for myself) how does a class C address use a mask that class B owns? ... In subnetting, it is obvious to borrow bits from the host portion to create new subnets. In the case here, a class C address is using a mask that enables us to play in the third octet. Third octet is always the game that class B addresses play.  How does a class C address use the third octet ? ... The only thing i saw that uses class C address with /21 mask is, Summarization. 

By the way, my understanding in subnetting is good, but maybe there are things in subnetting that i still don't know.

 

Thanks

Highlighted
Enthusiast

Yes, the main reason was

Yes, the main reason was increasing the number of hosts but i don't want to discuss their reason now. I want to know (for myself) how does a class C address use a mask that class B owns? ... In subnetting, it is obvious to borrow bits from the host portion to create new subnets. In the case here, a class C address is using a mask that enables us to play in the third octet. Third octet is always the game that class B addresses play.  How does a class C address use the third octet ? ... The only thing i saw that uses class C address with /21 mask is, Summarization. 

By the way, my understanding in subnetting is good, but maybe there are things in subnetting that i still don't know.

 

Thanks

Highlighted
Hall of Fame Guru

how does a class C address

how does a class C address use a mask that class B owns?

It isn't owned by any class.

You are thinking in classful terms and you need to think in classless terms.

If every device is configured with a /21 subnet mask ie. 255.255.248.0 then it simply means the range is -

network 192.168.0.0
useable IPs - 192.168.0.1 -> 192.168.7.254
broadcast IP - 192.168.7.255

that is all it means and there is nothing special about this.

Whether it is a good idea is another matter because presumably if they are using a single IP subnet with that many hosts in it then it is also one vlan which is a rather large broadcast domain.

Jon

 

Highlighted
Enthusiast

Jon,Let me calculate this./21

Jon,

Let me calculate this.

/21 gives us 5 bits for subnets and 11 bits for hosts.

 

2^5-2= 30 subnets.

2^11-2= 2046 hosts

 

256-248= 8

 

So, the valid subnets are: 0, 8, 16, 24, 30,....240

 

How is the IP range is 192.168.0.1 to 192.168.7.254?

Highlighted
Hall of Fame Guru

If the valid subnets go up in

If the valid subnets go up in multiples of 8 as you have said then the first subnet has to be -

192.168.0.0 and the next subnet is 192.168.8.0 which means everything in between is a useable IP address except 192.168.7.255 which is the broadcast.

I am using the term subnet here but technically it is a supernet.

Jon

Highlighted
Enthusiast

Then, the next IP range is:

Then, the next IP range is: 192.168.8.1 --> 192.168.15.254 ?

Highlighted
Hall of Fame Guru

Correct.Jon

Correct.

Jon

Highlighted
Enthusiast

Jon, thank you for your great

Jon, thank you for your great explanations and patience.

 

Now i realize that these guys are crazy to assign just one subnet (2046 hosts/subnet) and wasted 29 subnets with 59334 hosts. Believe it or not, they only got 120 hosts. :))


 

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