the-longest-single-dns-label-you-can-register-right-now-is-63ch.com

The part before .com is exactly 63 characters — the maximum length of a single DNS label.

What is the maximum length?

A DNS name is a sequence of labels separated by dots. On the wire there are no dots: each label is stored as one length byte followed by its bytes, and the name ends with a zero byte for the root.

• One label: 63 octets. Letters, digits, hyphens only. DNS cannot carry accents or non-Latin scripts, so a name like café.com is transliterated into plain ASCII before it is sent — xn--caf-dma.com. That encoding is called punycode, the xn-- prefix marks it, and the result still has to fit in 63 octets, which is why emoji and Chinese domains run out of room much sooner than they look like they should.
• Whole name: 255 octets on the wire — length bytes, label bytes, and the root zero, all included.
• 253 characters in the dotted form you type, because the framing bytes eat the difference.

Worked example of a maximal name. Four labels of 63, 63, 63, 61:

• label octets: 63 + 63 + 63 + 61 = 250
• length bytes: 4
• root zero: 1
• total: 255 — and 253 characters once you swap the length bytes for three dots

Sixty-three characters sounds cramped, so count what it buys. A label is 36 possible characters at each end (no leading or trailing hyphen) and 37 in the middle, over lengths 1 to 63:

• 6.1 × 10⁹⁸ possible names under a single TLD — a 99-digit number.
• 8.5 × 10¹⁰¹ across the ~1,400 TLDs in the root, ignoring subdomains entirely.

There are about 10⁸⁰ atoms in the observable universe. One TLD on its own has room for six billion billion domain names per atom. Roughly 350 million are registered, which is 10^-91 of .com-shaped space: the namespace is not running out, the pronounceable part of it is.

Why these limits?

RFC 1035, not aesthetics. The label length byte only has values 0–63 available: the top two bits are reserved for message-compression pointers, which is why 63 is a hard ceiling rather than a suggestion. 255 is simply the size of the domain-name field in that same message format. Every resolver, authoritative server, and TLS stack written in the last forty years assumes those bounds, so raising them is not on the table.

Longest name allowed

253 characters, e.g. three 63-character labels and a 61-character label:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

Technically the longest registrable domain — one label plus a TLD, no subdomains — wants the longest TLD you can actually buy under. That is .international at 13 characters. (Longer strings exist in the root, like .travelersinsurance, but they are closed brand TLDs.) So the ceiling for a name a member of the public can register in one go is 63 + 1 dot + 13 = 77 characters, versus 67 for the .com version of this site.

Swapping in .international also changes the shape of a maximal 253-character name: the TLD costs 14 octets on the wire, so you get three 63-character labels and then a 47-character one before you hit 255.

Caveat: the protocol maximum is not a promise. Plenty of registrars, browsers, and form validators impose stricter limits of their own.