Every network interface has a MAC address whose first half identifies the organization the IEEE assigned it to. Find a manufacturer's prefixes, or look up who owns a prefix.
18,720 organizations hold 34,841 MA-L blocks. Have a full MAC address? Use the MAC address vendor lookup.
A MAC address is 48 bits, written as six bytes such as 3C:22:FB:12:34:56. The IEEE Registration Authority sells blocks of addresses to manufacturers, and the organization then numbers its own devices inside the block:
Large vendors hold hundreds of MA-L blocks because each one covers about 16.7 million addresses. MA-M and MA-S blocks are carved out of MA-L blocks registered to "IEEE Registration Authority".
If the second-lowest bit of the first byte is set (the first byte's second hex digit is 2, 6, A or E), the address is locally administered: it was made up by software rather than taken from an IEEE block. Phones and laptops use such randomized MAC addresses on Wi-Fi for privacy, and virtual machines and containers often get them too. Those addresses never match a vendor. Some organizations also register blocks as "Private", which hides the owner.
Source: IEEE Registration Authority public listings (MA-L oui.txt). Organization names and addresses are shown as the IEEE publishes them.