TCP/IP

An IPv4 address is just a 32-bit number, written as four dot-separated bytes (192.168.1.42). A CIDR block like 192.168.1.0/24 splits those 32 bits into a fixed network prefix (the first 24 bits here) and a variable host portion (the remaining 8 bits) — every address sharing that same network prefix is "in the subnet."

def ip_to_int(ip):
    parts = [int(p) for p in ip.split(".")]
    result = 0
    for p in parts:
        result = (result << 8) | p
    return result

def in_subnet(ip, cidr):
    base, prefix_len = cidr.split("/")
    prefix_len = int(prefix_len)
    mask = (0xFFFFFFFF << (32 - prefix_len)) & 0xFFFFFFFF
    return (ip_to_int(ip) & mask) == (ip_to_int(base) & mask)

Once a packet is routed to the right subnet, TCP still needs to establish a reliable, ordered connection on top of IP's best-effort delivery. It does this with the three-way handshake: the client sends SYN (synchronize, "let's talk, my starting sequence number is X"), the server replies SYN-ACK (acknowledging X, plus its own starting sequence number Y), and the client replies ACK (acknowledging Y). Only after all three legs complete can either side send actual application data — which is exactly why a slow or dropped handshake (e.g. a firewall silently dropping the SYN) shows up as a hung connection rather than an instant error.