/30 subnet — mask 255.255.255.252

/30 means the first 30 bits are the network and 2 are left for hosts. That is 4 addresses, with mask 255.255.255.252.

Addresses are 32 bits, so the mask is written in four dotted parts: 255.255.255.0.

Seen as bits

The leading ones are the network part and the trailing zeros the host part. Add one more one and the network splits in two, halving the addresses.

/24 means the first 24 bits are the network; the remaining 8 hold the addresses inside it.
Each extra bit splits the network in two and halves the addresses.
Usable addresses are normally the total minus two — the network and broadcast addresses.
IPv6 has no broadcast, so nothing is subtracted. One network is conventionally a /64.

Q. How many addresses are in a /30?

4, of which 2 can actually be handed out.

Q. What is the subnet mask for /30?

255.255.255.252, with wildcard 0.0.0.3.

Q. How many /24 networks fit in a /30?

None — it is narrower than a /24.

Q. How many host bits are there?

2. Each extra network bit halves the addresses.