IP subnetting, okay to use the "zero" subnet?

I'm resubnetting a class B network (call it 129.1.x.x). With a mask of, the first usable range is thru (4094 hosts).

Historically this range has been considered the "zero" network, and not to be used. Does this mean is CAN'T be used, SHOULDN'T be used, or it's now OK to use.

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It is a valid network number.  Routers aren't going to treat it any differently than any other network number.  Unless of course, it recognizes the network:)
Close, while it is a subnet zero is a valid network, Cisco routers will certainly not allow you to assign an interface to the "zeroth" network unless you've included "ip subnet-zero" in your router config. Other than that RobWMartin's statement is correct, routers and switches will allow the use of subnet zero.
Key word is "historically" - you answered your own question!!!
I've got a T1 with PacBell...  They assigned me a 0 subnet.  So if you are taking a Microsoft test, no it can't be used, but in every other instance it can.

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