About macs4days
macs4days turns messy network output into a readable table of MAC addresses, registered prefix holders, and useful address classifications.
What this is
Paste a switch MAC address table, ARP output, a DHCP lease file, a log, or a CSV export exactly as your device printed it. macs4days pulls out every MAC address it can find along with the details sitting next to it: IP address, hostname, VLAN, type, age, and interface. Each address is classified and, where appropriate, matched against the IEEE MAC assignment registries so a string of hex becomes useful evidence rather than a guess about the device.
There is no cleanup step and no required format. The parser is built around what real equipment prints, which is rarely tidy and never consistent between vendors.
Why it exists
Identifying unfamiliar devices on a network usually means copying MAC addresses out of terminal output one at a time, pasting each into a lookup site, reading the vendor, and going back for the next one. Fifty unknown addresses is an afternoon. macs4days takes the whole paste at once and does not ask you to reformat anything first.
How vendor lookups work
IEEE MAC assignment prefixes can identify the organization that registered an address block. macs4days carries the MA-L, MA-M, MA-S, and IAB registries locally and refreshes them daily so newly registered prefixes resolve. A match is useful attribution evidence, not proof of who manufactured or operates a particular device.
Locally administered unicast addresses are marked with their IEEE 802c SLAP quadrant. Only the Extended Local Identifier (ELI) quadrant is checked against the separate CID registry. When its exact first 24 bits match, macs4days names the CID prefix registrant separately from the hardware vendor. That identifies who registered the number range, not who generated the address or made the device; randomized addresses can coincide with registered CID prefixes. Other local quadrants and multicast addresses are classified without guessing an organization.
Nothing you paste leaves your browser
Parsing and vendor lookups run entirely in your browser, against the copy of the database stored on your own machine. The text you paste is never uploaded, and there is no analytics or tracking script on this site. Results leave your machine only when you choose an action that sends them somewhere.
The privacy policy covers the full detail, including how advertising works and what is stored locally.
It works offline
The vendor database downloads once, roughly 2.3MB, and then keeps itself up to date in the background. After that first download, lookups keep working with no connection at all. You can install macs4days as an app, which is the point of building it this way: the moment you need to identify a device is often the moment you are standing in a rack room with no usable wifi.
Who runs it
macs4days is built and maintained by Thirsty Werk, one independent network engineer. It is not affiliated with the IEEE or with any hardware vendor whose prefixes it looks up: the registry data is public, and everything built around it here is one person's work. That is also why corrections are worth sending.
Corrections and contact
If a lookup is wrong, or output from a device you use does not parse cleanly, that is worth reporting: real-world samples are what the parser is built from. Use the contact option in the footer of this page.