Validating addresses and checking the same subnet

Why validation matters

Addresses are typed by people into forms, configuration files, firewall rules and scripts. A single typo, 192.168.1.300 or 10.0.0, can break a deployment or open a hole in a firewall. Any tool you write that takes an IP address should check it first and give a clear error.

The rules for a valid IPv4 address

1. Exactly four parts separated by dots. 10.0.0 and 1.2.3.4.5 are invalid.
2. Each part is made only of the digits 0 to 9. No letters, no spaces, no plus or minus signs, and no empty parts like 1..2.3.
3. Each part is between 0 and 255.
4. No leading zeros: 010.1.1.1 is rejected. Some old tools read a leading 0 as an octal (base 8) number, so 010 meant 8, not 10. Modern Python refuses leading zeros to avoid that confusion.

import ipaddress

for text in ["192.168.1.10", "192.168.1.300", "10.0.0", "010.1.1.1"]:
    try:
        print(text, "->", ipaddress.ip_address(text))
    except ValueError as error:
        print(text, "-> invalid:", error)

Are two hosts in the same subnet?

A PC sends a packet directly to a destination when the destination is in its own subnet, and to the default gateway when it is not. The test is the AND from earlier: AND both addresses with the mask. If the results are equal, they are in the same subnet.

With the block size method you do it in your head. Find the block that each address falls into. Same block, same subnet.

• 192.168.1.10 and 192.168.1.70 with /26: block size 64. 10 is in 0 to 63 and 70 is in 64 to 127. Different subnets, so they need a router to talk.
• 10.0.5.20 and 10.0.6.30 with /23: block size 2 in the third octet. 5 is in 4 to 5 and 6 is in 6 to 7. Different subnets.
• 172.16.20.1 and 172.16.31.254 with /20: block size 16 in the third octet. Both 20 and 31 are in 16 to 31. Same subnet.

import ipaddress

net = ipaddress.ip_network("192.168.1.10/26", strict=False)
print(net)                                          # 192.168.1.0/26
print(ipaddress.ip_address("192.168.1.70") in net)  # False
print(ipaddress.ip_address("192.168.1.40") in net)  # True

With strict=True (the default), ip_network("192.168.1.10/26") raises ValueError, because 192.168.1.10 has host bits set and is not a network address. That is Python telling you that you typed a host where a network was expected.

Common mistakes in real networks

Host address used as a network. Someone writes a route or firewall rule for 10.4.7.25/24 when they meant 10.4.7.0/24. Some devices quietly fix it, others reject it, and others store something unexpected. Always write the real network address.

Wrong mask on one PC. PC A is 192.168.1.10/24 and PC B is 192.168.1.200 but was given /25 by mistake. A thinks B is local and sends to it directly. B works out that A (in 192.168.1.0 to 127) is not in its subnet (192.168.1.128 to 255) and sends replies to the default gateway instead. Traffic may work one way, through the router, or not at all. Symptoms that "half work" are a strong hint to check masks.

Duplicate IP addresses. Two devices have the same address, often a static address typed by hand that is also in the DHCP pool. Both fight over it: connections drop at random, and Windows may show an "IP address conflict" warning. Fix it by keeping static addresses outside the DHCP range and recording every static address in an IP plan.