Let me clarify the terms all ones and zero subnet. Remember that an IP address is made up of three parts, the network portion, the subnet portion (optional), and the host portion. Routers determine the difference by use of the subnet mask.
The host portion is not used for routing. However, it is the host portion that determines the type of address. When the host portion is all ones it the broadcast address. When the host portion is all zeros it the network or subnet address. The host portion hast to have at least one 1 and at least one 0 to be in the range of usable addresses for configuring hosts.
The subnet portion, when used, can be all zeros or all ones and that is what is referred to as the zero or all ones subnet. On Cisco routers with older IOS’s you have to use the “ip subnet-zero” command to enable use of the zero and all ones subnets. On Cisco routers with newer IOS’s the command is enabled by default.
Hope this helps. Subnetting is so much fun. :lol:
