BGP Community Builder
Communities are tags carried with a route. They mean whatever the network carrying them has decided they mean — with a handful of exceptions that are standardised.
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Standard, large, and what they actually mean
A standard community is 32 bits, written as two 16-bit halves —
65000:100. Convention puts the ASN first and a value the ASN chose second.
A large community is 96 bits in three 32-bit parts —
65000:1:200. It exists because 32-bit ASNs do not fit in a 16-bit half, which made
standard communities unusable for a growing share of the internet.
Most values mean nothing universally. 65000:100 means whatever
AS 65000 published that it means, usually in a document on their website. Do not assume a
community you have seen elsewhere carries the same meaning here.
A few are standardised. no-export (65535:65281) asks that the
route not be advertised outside the receiving AS; no-advertise (65535:65282) asks
that it not be advertised to any peer at all. These are honoured broadly, though honouring them
is still a policy decision at each network.
Communities are how customers signal intent to a provider — prepend here, do not export there, set a local preference. What each provider accepts is entirely up to them.
The well-known values, and why they are the exception
A standard community is 32 bits, written as two 16-bit halves — conventionally the ASN that defines the meaning, then a value that ASN has assigned. The meaning lives in the operator's policy and nowhere else, which is why a community that makes a route less preferred on one network may do nothing at all on the next.
A handful are reserved and behave the same everywhere, because they are defined by the protocol rather than by a policy. NO_EXPORT (65535:65281) tells a receiving AS not to advertise the route outside its own confederation or AS boundary. NO_ADVERTISE (65535:65282) says not to pass it to any peer at all. NO_EXPORT_SUBCONFED (65535:65283) stops it leaving the local sub-autonomous-system. These are worth recognising on sight, because they are the only ones you can reason about without reading somebody's documentation.
Large communities exist because of four-byte ASNs
The original format has 16 bits for the ASN half, which was fine until AS numbers grew to 32 bits. An operator with a four-byte ASN cannot fit it into the first half of a standard community at all, so the convention of "my ASN, then my value" simply stops working. Various uncomfortable workarounds followed.
RFC 8092 defines large communities to settle it: 96 bits in three 32-bit fields, written global:local1:local2. The first field holds a full four-byte ASN with room to spare, and the two remaining fields let an operator express both a function and its target — "prepend twice", "towards this ASN" — which the old format never had space for. If you are designing a community scheme now, this is the format to design it in.
Related tools
AS-Path Visualizer and the Best-Path Simulator cover the attributes that decide routing.