Why 192.168.1.1/24 Pings 192.168.1.2/23 Despite Different Network IDs

The author discovered a puzzling scenario: 192.168.1.1/24 (network 192.168.1.0) and 192.168.1.2/23 (network 192.168.0.0) have different network IDs but can ping each other; conversely, 192.168.1.2/23 and 192.168.0.1/24 share the same network ID ( 192.168.0.0) but cannot ping. The root cause is a common misconception: people apply human logic ("I think...") instead of the computer's perspective ("the computer thinks...").

1. Why Different Network IDs Can Communicate

When 192.168.1.1 pings 192.168.1.2, the computer first checks if the target is in the same subnet using its own subnet mask (/24) . It performs a bitwise AND between its IP ( 192.168.1.1) and mask ( 255.255.255.0) to get its network ID ( 192.168.1.0). Then it applies the same mask to the target IP ( 192.168.1.2) and gets the same network ID ( 192.168.1.0). Since they match, it ARPs for the target's MAC and communicates directly. The key point: the computer never uses the target's mask (/23); it only uses its own mask. This is emphasized three times.

2. Binary Calculation Walk‑through

IP = 192.168.1.1 = 11000000.10101000.00000001.00000001
Mask = 255.255.255.0 (/24) = 11111111.11111111.11111111.00000000
Network ID = 11000000.10101000.00000001.00000000 = 192.168.1.0

Target IP = 192.168.1.2 = 11000000.10101000.00000001.00000010
Mask (same /24) = 11111111.11111111.11111111.00000000
Target Network ID = 11000000.10101000.00000001.00000000 = 192.168.1.0

Both network IDs are identical ( 192.168.1.0), so the hosts are considered on‑link. All calculations are from 192.168.1.1 's perspective only.

3. General Principle: You Never Need the Remote Host's Mask

You cannot know the mask of a remote IP like 8.8.8.8 (it could be /32, /31, /8, etc.), yet you can still reach it. Your computer uses only its local mask to decide: if the destination's network ID (computed with your mask) matches yours, send directly via ARP; otherwise, forward to the configured gateway. Using the remote host's mask is the fundamental error — it is your human perspective, not the computer's.

4. Why Same Network ID Fails to Communicate

From 192.168.0.1/24 's perspective: its network ID is 192.168.0.0 (using its /24 mask). It calculates the network ID of 192.168.1.2 using that same /24 mask, yielding 192.168.1.0. The two network IDs differ, so the computer determines the target is remote and requires a gateway. With no gateway configured, the packet is dropped — hence the ping fails.

Conclusion: Always reason from the computer's viewpoint — apply the local subnet mask to both local and remote IPs — to avoid subnet‑mask confusion.