192.168.4.0 – 192.168.4.255 is the correct answer.
The wildcard mask 0.0.0.255 matches all packets that have the same first three octets. In this case, the ACL rule applies to the range:
192.168.4.0 - 192.168.4.255
Wildcard masks use two simple rules:
When the decimal is 0 then the router has to compare the octet.
When the decimal is 255 then the router has to ignore the octet.
In this case, we have to find the range of the network 192.168.4.0 matched by the command access-list 2 permit 192.168.4.0 0.0.0.255
As you can see the wildcard mask is 0.0.0.255 which means that the packets that come from the network 192.168.4 (firs three octets) have to be compared once reach the router, and the 0 (last octet) has to be ignored. The process is as follow:
A packet with IP address 192.168.4.20 reaches the router. The router then starts asking (compare), are the first three octets of the packet 192.168.4?
If so, then there is a match (it's a match). If not, then there isn't a match.
A new packet with IP address 192.168.2.20 reaches the router.
The router then starts asking (compare), are the first three octets of the packet 192.168.4?
If so, then there is a match. If not, then there isn't a match (it's not a match).
As a result, the exact range of IP addresses, matched by the command is: 192.168.4.0 - 192.168.4.255
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