addrv2 (BIP155)

BIP155 defines addrv2, a peer-to-peer message Bitcoin nodes use to share known peer addresses. It replaces the limitations of the legacy addr message, whose fixed 16-byte address field could not represent the longer endpoint formats used by modern privacy and overlay networks. Address gossip is how nodes discover new peers, so supporting these networks is essential to keeping the network reachable over privacy-preserving transports.

Networks it can carry

The addrv2 format introduces an 8-bit network identifier and a variable-length address field, allowing it to encode IPv4 (4 bytes), IPv6 (16 bytes), Tor v3 onion services (32-byte ed25519 keys), I2P (32-byte SHA-256-based destinations), CJDNS (IPv6 in the fc00::/8 range), and Yggdrasil. The obsolete 10-byte Tor v2 format is still representable but is no longer operational. Service bits are encoded as a CompactSize integer rather than a fixed 8 bytes.

Negotiation and compatibility

Peers signal support by exchanging a sendaddrv2 message during the version handshake; only after both sides advertise it do they use the new format. Legacy peers continue to receive the old addr message, so the upgrade is backward compatible. By making Tor v3, I2P, and CJDNS first-class citizens of address gossip, BIP155 lets nodes run reachably on those networks instead of being limited to clearnet IP.

Reaching peers privately pairs naturally with connection encryption, see BIP324 (v2 Encrypted P2P Transport), and with relay-layer privacy, see Dandelion (BIP156).