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Subnet Zero Help

FloOz

This is driving me crazy. I am having trouble understanding what exactly is the point of subnet zero. I understand that the subnet zero is equal to the first subnet of an address. What exactly is the use of the subnet zero command? I swear I am over thinking this. Any help is greatly appreciated.

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Comments

Ltat42a

Look at it this way. You have the network 192.168.0.0/24, and you need to subnet that into smaller subnets. We'll use 192.168.0.0/27.
Using this subnet mask (255.255.255.224), your networks will increment by 32.
#1 192.168.0.0
#2 192.168.0.32
#3 192.168.0.64
#4 192.168.0.96
etc
etc
etc

By not using subnet zero, you cannot use the #1 network (192.168.0.0), a waste of 30 available IP addresses.

With some of the older IOS, subnet zero was disabled by default, so to enable it, you had to isssue the ip subnet zero command in order to use that first network of addresses. With the newer IOS's, the subnet zero is on by default.

hth

prtech

Older IOSs didn't allow you to configure interfaces with an IP address that belonged to subnet zero until you issued the "ip subnet-zero" command.

FloOz

Ltat42a wrote: »

Look at it this way. You have the network 192.168.0.0/24, and you need to subnet that into smaller subnets. We'll use 192.168.0.0/27.
Using this subnet mask (255.255.255.224), your networks will increment by 32.
#1 192.168.0.0
#2 192.168.0.32
#3 192.168.0.64
#4 192.168.0.96
etc
etc
etc

By not using subnet zero, you cannot use the #1 network (192.168.0.0), a waste of 30 available IP addresses.

With some of the older IOS, subnet zero was disabled by default, so to enable it, you had to isssue the ip subnet zero command in order to use that first network of addresses. With the newer IOS's, the subnet zero is on by default.

hth

Thank you so much!

FloOz

One more quick question. Would there ever be a need to actually disable subnet zero?

Ltat42a

In a real life production network, I would guess - no. If it were to appear on an exam question, yes, you may need know that (no ip subnet zero).

hth

NetworkVeteran

FloOz wrote: »

One more quick question. Would there ever be a need to actually disable subnet zero?

No. If you don't want anyone using subnet zero on your networks, you could also create and enforce a policy of not using it. This command isn't your only option.