Break out the discussion of special and reserved IPv4 addresses into a subsection, formatted as a pair of definition lists, and briefly describing three cases in which Linux no longer treats addresses specially, where other systems do or did. Also add a specific example to the NOTES paragraph that discourages the use of IP broadcasting, so people can more easily understand what they are supposed to do instead. --- man7/ip.7 | 83 +++++++++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 75 insertions(+), 8 deletions(-) diff --git a/man7/ip.7 b/man7/ip.7 index 6c50d0281..6f1ee4dbe 100644 --- a/man7/ip.7 +++ b/man7/ip.7 @@ -237,19 +237,82 @@ In particular, this means that you need to call on the number that is assigned to a port. All address/port manipulation functions in the standard library work in network byte order. -.PP +.SS Special and reserved addresses There are several special addresses: -.B INADDR_LOOPBACK -(127.0.0.1) +.TP +.BR INADDR_LOOPBACK " (127.0.0.1)" always refers to the local host via the loopback device; +.TP +.BR INADDR_ANY " (0.0.0.0)" +means any address for socket binding; +.TP +.BR INADDR_BROADCAST " (255.255.255.255)" +has the same effect on +.BR bind (2) +as .B INADDR_ANY -(0.0.0.0) -means any address for binding; +for historical reasons. +A packet addressed to .B INADDR_BROADCAST -(255.255.255.255) -means any host and has the same effect on bind as +through a socket which has +.B SO_BROADCAST +set will be broadcast to all hosts on the local network segment, +as long as the link is broadcast-capable. + +.TP +Highest-numbered address +.TQ +Lowest-numbered address +On any locally-attached non-point-to-point IP subnet +with a link type that supports broadcasts, +the highest-numbered address +(e.g., the .255 address on a subnet with netmask 255.255.255.0) +is designated as a broadcast address. +It cannot usefully be assigned to an individual interface, +and can only be addressed with a socket on which the +.B SO_BROADCAST +option has been set. +Internet standards have historically +also reserved the lowest-numbered address +(e.g., the .0 address on a subnet with netmask 255.255.255.0) +for broadcast, though they call it "obsolete" for this purpose. +(Some sources also refer to this as the "network address.") +Since Linux 5.14, +.\" commit 58fee5fc83658aaacf60246aeab738946a9ba516 +it is treated as an ordinary unicast address +and can be assigned to an interface. + +.PP +Internet standards have traditionally also reserved various addresses +for particular uses, though Linux no longer treats +some of these specially. + +.TP +[0.0.0.1, 0.255.255.255] +.TQ +[240.0.0.0, 255.255.255.254] +Addresses in these ranges (0/8 and 240/4) are reserved globally. +Since Linux 5.3 +.\" commit 96125bf9985a75db00496dd2bc9249b777d2b19b +and Linux 2.6.25, +.\" commit 1e637c74b0f84eaca02b914c0b8c6f67276e9697 +respectively, +the 0/8 and 240/4 addresses, other than .B INADDR_ANY -for historical reasons. +and +.BR INADDR_BROADCAST , +are treated as ordinary unicast addresses. +Systems that follow the traditional behaviors may not +interoperate with these historically reserved addresses. +.TP +[127.0.0.1, 127.255.255.254] +Addresses in this range (127/8) are treated as loopback addresses +akin to the standardized local loopback address +.B INADDR_LOOPBACK +(127.0.0.1); +.TP +[224.0.0.0, 239.255.255.255] +Addresses in this range (224/4) are dedicated to multicast use. .SS Socket options IP supports some protocol-specific socket options that can be set with .BR setsockopt (2) @@ -1343,6 +1406,10 @@ with careless broadcasts. For new application protocols it is better to use a multicast group instead of broadcasting. Broadcasting is discouraged. +See RFC 6762 for an example of a protocol (mDNS) +using the more modern multicast approach +to communicating with an open-ended +group of hosts on the local network. .PP Some other BSD sockets implementations provide .B IP_RCVDSTADDR -- 2.25.1
