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Verify Subnet zero

Hi. I just need to clarify if ip subnetzero is enabled, does that mean i can use subnetworks as ip addresses like 192.168.0.0 for example?

Thanks in advance.

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Hall of Fame Super Blue

Re: Verify Subnet zero

No that's not what ip subnet-ero means.

192.168.0.0 255.255.255.0 = subnet 192.168.0

You can use this subnet but 192.168.0.0 is not an address, it is a subnet number so the first address would be

192.168.0.1 255.255.255.0

You generally wouldn't want to allocate another IP out of the same subnet to another interface on the same router - with ethernet it will not let you, with serial i seem to remember reading on this forum that the router will let you do this but not sure why you would want to.

Jon

10 REPLIES
Hall of Fame Super Blue

Re: Verify Subnet zero

Assuming 192.168.0.0/24 yes it does mean that.

Jon

New Member

Re: Verify Subnet zero

Just to clarify, does that mean that i can assign an ip add of 192.168.0.0/24 on one serial int and 192.168.0.1/24 on another attached serial interface within the same subnet which is 192.168.0.0/24?

Hall of Fame Super Blue

Re: Verify Subnet zero

No that's not what ip subnet-ero means.

192.168.0.0 255.255.255.0 = subnet 192.168.0

You can use this subnet but 192.168.0.0 is not an address, it is a subnet number so the first address would be

192.168.0.1 255.255.255.0

You generally wouldn't want to allocate another IP out of the same subnet to another interface on the same router - with ethernet it will not let you, with serial i seem to remember reading on this forum that the router will let you do this but not sure why you would want to.

Jon

Silver

Re: Verify Subnet zero

Hi Jon

I just checked using Dynamips, and you are correct. A router will except IP addresses from the same subnet/network on serail interfaces.

Well blow me down with a feather!

Best Regards,

Michael

Silver

Re: Verify Subnet zero

Hi Marcus

No, I don't think a router will not allow you to configure two separate interfaces with IP addresses from the same subnet or over lapping subnets (i.e. 192.168.0.1/16 and 192.168.1.1/24) for that matter.

***Edit*** This statement is not strictly correct, as a router will allow you to configure IP addresses from the same subnet on Serial interfaces. The above statement only applies to Ethernet interfaces. Thanks Jon *** End of Edit ***

Also having "ip subnet zero" enabled has nothing to do with configuring a subnet number on an interface. Whether "ip subnet zero" is enabled or not it is considered bad design practice to assign either the network/subnet number or the broadcast IP address to any interface, whether the equipment will accept this configuration or not.

The algorithm for working out the usable host IP addrsses on a subnet is (h^2)-2, where "h" equals the number of host bits (contiguous "0's") in the subnet mask, reading in binary from right to left. On a /24 subnet this figure is 254 or (8^2) = 256 -2 = 254.

Enabling "ip subnet zero" allows the use of the first and last subnets when subnetting a major network.

E.g

If you had the major network 152.16.0.0/16 and you want to subnet it using a /24 bit mask.

With "ip subnet zero" enabled the algorithm to work out the number of subnets is n^2, where "n" is equal to the number of host bits borrowed. 8^2=256, so you have 256 usable subnets.

With "ip subnet zero" disabled, the algorithm is (n^2)-2, where "n" is equal to the number of hosts bits borrowed. (8^2) = 256 -2 = 254, so you can only use 254 subnets.

HTH

Best Regards,

Michael

New Member

Re: Verify Subnet zero

Jon, sorry to disturb you again. I got a bit confused on the subnet issue. Given an ip of 192.168.0.0/24, the total no. of subnets according to the formula is 254 right? But the actual subnet range would be from 192.168.0.0/24 to 192.168.254.0/24 isn't it?

Then the total would be 255. Is this right?

Hall of Fame Super Blue

Re: Verify Subnet zero

No problem.

192.168.0.0/24 = 192.168.0.0 255.255.255.0 = 1 subnet with 254 possible host addresses

Where your confusion comes from is the subnet mask ie. /24.

192.168.0.0/16 = 192.168.0.0 255.255.0.0 is what you are thinking of.

Jon

New Member

Re: Verify Subnet zero

But considering that i intend to use 192.168.0.0/24, the subnet range would be 192.168.0.0/24 till 192.168.254.0 right? Which is a total of 255 subnets?

Hall of Fame Super Blue

Re: Verify Subnet zero

Okay i see the confusion. Yes your subnets would be

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

which is summarisable as 192.168.0.0/16.

Does this make sense ?

Jon

New Member

Re: Verify Subnet zero

Thanks for your patience Jon. I guess i was referring to your previous statement where you said:

With "ip subnet zero" enabled the algorithm to work out the number of subnets is n^2, where "n" is equal to the number of host bits borrowed. 8^2=256, so you have 256 usable subnets.

Considering that i intend to use 192.168.0.0/24, does that mean that aside from being able to use 192.168.0.0/24 as a subnet, i can also use 192.168.255.0/24 as a subnet? If this is the case, the broadcast address of subnet 192.168.255.0/24 is 192.168.255.255 right? I think i get it now.

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