What is the setting for Cisco ip subnet-zero switches?

Question

Cisco

I apologize for the noob question.
Met in many configs such line
ip subnet-zero
What does it mean?

Answer 1

I will add to what has already been said.
The command makes sense for classful allocation of addresses. In the example given by Valentine , it turns out that the class network and its "zero" subnet have the same number (192.168.1.0). Apparently for this reason RFC950 recommends:
Classful routing works like this:
1) when a packet is received with a destination address, for example, 192.168.1.1, its (classic) network number, 192.168.1.0, is determined.
2) the router checks if there is a network in the routing table
3) the router checks if the subnet (192.168.1.0/27) corresponding to the destination address is in the routing table and forwards the packet according to the next-hop entry.
The command ip subnet-zeroaffects step 3 of this process, if it is activated, the desired subnet will be present in the routing table and the packet will be redirected accordingly. Without this command, the packet will be dropped (if there is a classful network entry but no subnet entry).
In the current conditions, when routing is based on the search for the most exactly matching prefix (longest prefix match), the command is irrelevant, because. the problem it solves does not arise (192.168.1.0/24 and 192.168.1.0/27 are different prefixes, there is no confusion, and accordingly it makes no sense to prohibit the use of "zero subnets").