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API Quickstart 
curl https://networkcalc.com/api/ip/192.168.1.0/24

{
  "status": "OK",
  "address": {
    "cidr_notation": "192.168.1.0/24",
    "subnet_bits": 24,
    "subnet_mask": "255.255.255.0",
    "wildcard_mask": "0.0.0.255",
    "network_address": "192.168.1.0",
    "broadcast_address": "192.168.1.255",
    "assignable_hosts": 254,
    "first_assignable_host": "192.168.1.1",
    "last_assignable_host": "192.168.1.254"
  }
}

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Subnetting and DNS: Quick Answers

CIDR notation writes an IP address followed by a slash and the number of bits that identify the network, such as 192.168.1.0/24. A /24 means the first 24 of 32 bits are the network, which is the same as the subnet mask 255.255.255.0; the last 8 bits identify hosts. Try it in the subnet calculator or read the CIDR notation guide.

A subnet with n host bits holds 2n addresses, and for most subnets two of them are reserved: the network address and the broadcast address. A /24 has 256 addresses and 254 usable hosts; a /30 has 4 addresses and 2 usable hosts. The subnet calculator shows the first and last assignable host for any block, and what is subnetting? explains why networks are divided this way.

A DNS lookup shows the records published for a domain: A and AAAA records map a name to IPv4 and IPv6 addresses, MX records name the mail servers, NS records name the authoritative name servers, and TXT records hold text such as SPF policies. Each record has a TTL that tells resolvers how long to cache it, which is why DNS changes can take time to appear everywhere. Run one with the DNS lookup tool or the DNS API.

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