Connecting to a Classic server

Ports

Port Transport Used for
6112 TCP The Battle.net connection itself (BNCS), for every Classic game and the chat gateway
6112 UDP Connectivity tests between server and client, and peer-to-peer game traffic for Diablo, StarCraft and Warcraft II
6112 TCP Warcraft III game hosting (the player can change this port)
Given by the server TCP The Diablo II realm (MCP). The address and port arrive in SID_LOGONREALMEX.
4000 TCP Diablo II game servers. The client cannot be told to use another port.

The first byte: choosing a protocol

Right after the TCP connection opens, the client sends one byte that tells the server which protocol follows. The server does not answer this byte.

Byte Protocol that follows Confidence
0x01 BNCS, the game protocol. Diablo II also sends this on its realm connection. ✅ Verified
0x02 BNFTP, file transfer (version-check files, icons.bni, terms of service, advertisement banners) ✅ Verified
0x03 The chat gateway (telnet), now historical ✅ Verified

A server that receives any other first byte should close the connection without replying.

How messages are framed

Different protocols in the Classic family frame their messages differently. Mixing them up is the most common mistake when writing a client or server.

Protocol Header Length counts
BNCS FF, message ID (1 byte), length (2 bytes) the whole message, including its 4-byte header
MCP (Diablo II realm) length (2 bytes), message ID (1 byte). There is no FF, and the length comes first. the whole message, including its header
W3GS (Warcraft III games) F7, message ID (1 byte), length (2 bytes) the whole message, including its header
Chat gateway Text lines ending in CR LF —

An empty BNCS message, such as a keep-alive, is exactly four bytes: FF 00 04 00.

Data types

Every number is little-endian.

Type Size Meaning
UINT8, UINT16, UINT32, UINT64 1, 2, 4, 8 bytes Unsigned integers
STRING varies Text ending in a zero byte
STRINGLIST varies Several STRINGs, ending with an empty one
FILETIME 8 bytes A Windows file time: 100-nanosecond intervals since January 1, 1601
DWORD (four-character code) 4 bytes A code such as STAR or IX86, stored as a UINT32, so its letters appear reversed on the wire: STAR is sent as 52 41 54 53 (RATS)
VOID varies Raw bytes with no further structure

Text encoding and limits

  • StarCraft products (STAR, SEXP, SSHR, JSTR) send chat text as UTF-8. The other Classic games use ISO 8859-1.
  • Chat messages are limited to 224 bytes, including the terminating zero.
  • Channel names are limited to 31 characters, and account names to 15.

Sources and credits

Categories: Classic Battle.net
Last edited September 16, 2026 by John Leighow.
Edit on GitHub | Page history