subnet bits - all 1's

Why do they object to all zeros/all ones in the subnet bits?

Hi,
Say i have a subnet mask of
255.255.192.0 and my class B n/w addr is
130.120.0.0

My subnets are
130.120.0.0
130.120.64.0
130.120.128.0
130.120.192.0

why is it that 130.120.0.0
and 130.120.192.0 are forbidden unless you specifically use the ip subnet-zero cmd?

Thanks,
- K

Have a look at the post "IP Subnet Zero - Valid in CCNA Exam" in this forum