Member Avatar for Sphinx'LostNose

Can 2 identical IPs with different subnet masks be on the same LAN? I know you'd have to route to get between them, but is it acceptable to use the same set of IPs on 2 different subnets?

  • 5 Contributors
  • 5 Replies
  • 672 Views
  • 4 Weeks Discussion Span
  • Latest Post Latest Post by master73

Dani AI

Generated

No — two hosts on the same broadcast domain cannot legitimately share the exact same IPv4 address even if their subnet masks differ. raised the right question; the key idea from about ARP is the correct explanation, and the short replies from , and (after re-reading) are all aligned with that conclusion.

Why it breaks: each host uses its own mask only to decide whether a destination is “on‑link” or needs the gateway. If a destination is on‑link the sender issues an ARP request; ARP carries only the IP, not the mask. Two devices claiming 192.168.0.4 on the same LAN will both respond to ARP; ARP caches and the switch MAC table will flip to whichever MAC answered last, causing intermittent connectivity, packet loss and OS duplicate‑IP warnings. In this concrete case both 192.168.0.4/24 and 192.168.0.4/28 place .4 on‑link (192.168.0.0/28 = .0–.15; /24 = .0–.255), so a conflict is immediate.

When reuse is valid: identical addresses can be reused only when kept in separate broadcast domains or logically isolated — different VLANs, separate physical networks, VRF/network namespaces, or behind NAT. Proxy ARP can make remote addresses look local but does not permit two hosts on the same LAN to own the same IP.

Quick checks and common fixes: inspect ARP/neighbor entries and packet captures, check the switch MAC table and DHCP logs. Example commands:

arp -n
ip neigh show
tcpdump -n -i eth0 arp
# on switches: show mac address-table | include <MAC>

Resolutions include assigning unique addresses, aligning subnetting across hosts, using DHCP reservations, or intentionally isolating overlapping address spaces with VLANs, NAT or VRF.

Recommended Answers

All 5 Replies

Member Avatar for CimmerianX

Not going to work as you envision.

Member Avatar for rch1231

No. Will not work. It does not matter what subnet they are on, having the same IP would cause a conflict in the LAN's ARP table which says which MAC address is being referred to by the IP in question. When an address is referenced on the lan the subnet is not published with it. Subnets only tell you what addresses are local versus ones that you have to use the gateway to get to. The only way we are able to use the same IP's (public IP ranges 192.168.X.X or 10.0.X.X, etc.) is because those IP's are behind routers that translate the address (NAT) for us as packets are transmitted b ack and forth.

Member Avatar for Rakwl

Nope.

Member Avatar for master73

Sure it can be... If you create a route to that network.

Member Avatar for master73

Sphinx'LostNose- I do apologize and fully read your question. I now realize what you what to accomplish, and the answer is no.

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.