How can possible class c be connected the 300 pc in the same network?

class C, it can support a maximum of 254 devices, this excludes network address and broadcast address. If you are willing to accomodate 300 devices in a single network, then you must go for class B. But using class B for 300 devices would result in wastage of IP addresses. To mitigate this problem you can opt CIDR. The moment you start using CIDR, the network is no longer classful, it is classless. To use 300 devices you would need 9 host bits compared to the default 8 bits in class C. This will allow you to connect upto a maximum of 510 devices, thus minimizes the wastage of IP addresses to a greater extent. All you need to do is, configure the router with CIDR value of /23. The new subnet mask will be 255.255.254.0. As is it classless, you can use any address as network address, the first octet rule doesnt apply in CIDR