A BGP community is an attribute, attached to an announcement of a route to which the sender offers connectivity. Communities are most often identifiers for groups of routes/addresses to which some common policy applies.  There are both well-known communities that should be recognized by all BGP implementations, and various kinds of communities that are usually defined by an autonomous system 
To deal with Internet growth and the use of BGP in intranets and extranets (e.g., virtual private networks), various extended communities have been defined.  These primarily deal with internet operations issues, such as the scope of routing information distribution.
Basic structure of a community identifier
As first defined, a community is a 32-bit binary string, broken into two 16-bit fields. The first field's value is either all binary ones, indicating it is a "well-known" community, or contains the value of the autonomous system that defines the meaning of the second field.
By convention, a community is written:
ASN or 65535:specific meaning
Many router implementations will allow the ASN field to be displayed in decimal rather than hexadecimal, corresponding to general practice in BGP routing.
A BGP implementation supporting communities MUST understand the following well-known communities
|NO-EXPORT||Do not advertise this route outside the current autonomous system||FFFF:1|
|NO-ADVERTISE||Do not advertise this route to any other BGP router, even inside the same AS.||FFFF:2|
|NO_EXPORT_SUBCONFED||All routes received carrying a communities attribute containing this value MUST NOT be advertised outside a BGP confederation boundary (a stand-alone autonomous system that is not part of a confederation should be considered a confederation itself).||FFFF:3|
Since the ASNs from decimal 64512 through 65535 are reserved, these effectively are NO-EXPORT onto the Internet.
- Chandra R., Traina P., Li T. (August 1996), BGP Communities Attribute, RFC1997
- There are communities, typically used in intranets and extranets, where a prefix other than an autonomous system number is used to disambiguate
- Tappan D., Rekhter Y., Sangli I. (February 2006), BGP Extended Communities Attribute, RFC4360