-3 can be an address of network and it cannot be assigned to any device in local network. But why? Couldn't router using mask (like figure out what is the network address anyway?


From http://web.archive.org/web/20100821112028/http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf (page 14):

"Defining Host Addresses for Each Subnet According to Internet practices, the host number field of an IP address cannot contain all 0-bits or all 1-bits. The all-0s host number identifies the base network (or subnetwork) number, while the all-1s host number represents the broadcast address for the network (or subnetwork)."

More authoritative is RFC-943 at http://www.ietf.org/rfc/rfc943.txt:

In certain contexts, it is useful to have fixed addresses with
functional significance rather than as identifiers of specific
hosts.  When such usage is called for, the address zero is to be
interpreted as meaning "this", as in "this network".  The address
of all ones are to be interpreted as meaning "all", as in "all
hosts".  For example, the address could be
interpreted as meaning all hosts on the network 128.9.  Or, the
address could be interpreted as meaning host 37 on this
  • This restriction applies only to IPv4; IPv6 does not have a similar restriction, and the lowest network address is usable by hosts. – Michael Hampton Jan 12 '14 at 17:03
  • Wrong !. RFC 943 Obsoleted by RFC 960 Obsoleted by RFC 990 Obsoleted by RFC 1010 Obsoleted by RFC 1060 Obsoleted by RFC 1340 Obsoleted by RFC 1700 Obsoleted by RFC 3232. And the 3com document you linking is from 2001 !!! – Bartłomiej Zarzecki Jan 12 '14 at 17:41
  • @BartłomiejZarzecki - You're quite right. Unfortunately I can't delete an accepted answer. – HABO Jan 12 '14 at 17:55
  • Short version is: an IPv4 address with 0-s in the end is a network address. Long version at the proper university. – David Lakatos Jan 12 '14 at 19:07

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