MAC and IP addresses

[shaiko]

Hello,

Can the same IP address be shared between 2 MAC address?

 

[andre_luis]

At different networks shouldn't exists any apparent reason to impede doing that.
At the same network, unnecessary to say that cannot.

 

[shaiko]

Consider a PC with a WiFi and Wired LAN. 2 MACs - 1 IP. Am I correct?

 

[andre_luis]

Both conections could even be routed to the same subnet, but should necessarily have different IPs.

 

[FvM]

Both conections could even be routed to the same subnet

That's in fact the standard situation in a wired network with attached WLAN access points. A host with both interfaces connected to the network must have two IPs. Otherwise an ARP request would be answered by both interfaces with different MAC addresses, indicating an IP address conflict.

 

[bigdogguru]

Can the same IP address be shared between 2 MAC address?

The short answer:

Typically no, especially if the devices are located on the same subnetwork and the Internet Protocol (IP) is utilized.

The long answer:

The Internet Protocol (IP) makes use of the Address Resolution Protocol (ARP) to translate assigned IP addresses to the actual device's MAC address, or also commonly referred to as the hardware or physical address, to which the specific IP address is assigned. The ARP accomplishes this task by broadcasting a query (ARP request) requesting the device assigned a specific IP address respond (ARP reply) with its MAC address, which are then stored in a reference table within the ARP cache. The ARP cache has a specific Time To Live (TTL) duration which when expired, triggers the ARP cache to be flushed and aforementioned process to begin again.

Keeping in mind, IP communications are essentially established like so:

IP Address -> MAC Address ---------------> MAC Address -> IP Address

Rather than:

IP Address -------------------------> IP Address

Therefore, if the same IP address is assigned to two different Ethernet interfaces, i.e., two different MAC address, a conflict exists and typically triggers what is commonly referred to as ARP or MAC flapping, which can in turn can generate all sort of errors and timeouts at various levels of the network. For example, if such a conflict is detected on a system with a Windows OS, it will typically generate a warning or error message, both displaying it at the console level and logging it in the events logs.

Ethernet switches also make use of a similar technique, by building a MAC address to Port table, which in turn allows the switch to determine which Ethernet port to send packets destine for a particular MAC address, i.e., a specific device's Ethernet interface connected to that switch's port, as opposed to simply broadcasting those packets to all its Ethernet ports as would a Ethernet hub. The selectivity described is one of the primary differences between an Ethernet switch and an Ethernet hub.

You can examine the current ARP table on most OSs by issuing a command similar to arp -a, in a terminal or command window elevated to administrator privileges.


BigDog