Add a more detailed description of the IP addresses that have
a special meaning in Internet standards, and how these affect
Linux.
This revision notes details that network devices may not support
broadcasting at the link layer (and how to check).
Signed-off-by: Seth David Schoen <schoen@xxxxxxxxxxx>
---
man7/ip.7 | 39 +++++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)
diff --git a/man7/ip.7 b/man7/ip.7
index 7eee2811e..27df71ba4 100644
--- a/man7/ip.7
+++ b/man7/ip.7
@@ -233,6 +233,7 @@ 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)
@@ -245,6 +246,44 @@ means any address for binding;
means any host and has the same effect on bind as
.B INADDR_ANY
for historical reasons.
+.PP
+Internet standards have also traditionally reserved various
+addresses for particular uses. The addresses
+in the ranges 0.0.0.0 through 0.255.255.255 and 240.0.0.0 through
+255.255.255.254 (0/8 and 240/4 in CIDR notation) are reserved globally
+(but Linux permits addresses within these ranges, other than 0.0.0.0, to
+be assigned to an interface and used like other unicast addresses).
+All addresses in 127.0.0.0 through 127.255.255.255 ("127/8")
+are treated as loopback addresses akin to the standardized local
+loopback address 127.0.0.1, while addresses in 224.0.0.0 through
+239.255.255.255 ("224/4") are dedicated to multicast use.
+.PP
+On any locally-attached IP subnet that contains more than two IP
+addresses, the lowest-numbered address and highest-numbered address
+(e.g., the .0 and .255 addresses on a subnet with netmask 255.255.255.0)
+are both designated as broadcast addresses by default. These cannot
+usefully be assigned to an interface as its own address, and can only
+be addressed with a socket on which the
+.B SO_BROADCAST
+option has been explicitly enabled. The ability to actually send and
+receive broadcasts at the link layer is available on most transports
+(like Ethernet), but may not be implemented on some non-broadcast
+multiple access (NBMA) networks, such as some methods of sending IP
+over ATM. Individual network interfaces (see
+.BR netdevice (7))
+are designated as
+.BR IFF_LOOPBACK ,
+.BR IFF_BROADCAST ,
+.BR IFF_POINTOPOINT ,
+or, more rarely, none of these three (NBMA); only those designated
+.B IFF_BROADCAST
+possess the ability to send and receive broadcasts at the link
+layer. These flags can be accessed with
+.BR getifaddrs (3)
+or viewed with
+.BR ip-link (8)
+or
+.BR ifconfig (8).
.SS Socket options
IP supports some protocol-specific socket options that can be set with
.BR setsockopt (2)
--
2.25.1
