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Date: Mon Feb 03 2003 - 05:23:22 CST
I've put a lot of time into researching further into what I already know
about TCP/IP connections, and this is what I've learned:
The Well Known Ports are those from 0 through 1023.
The Registered Ports are those from 1024 through 49151
The Dynamic and/or Private Ports are those from 49152 through 65535
Originally, services ran on the Well Known Ports only (after all, 1024 is
more than enough, since no-one would ever need more than 640KB of memory,
would they?). In time these spilled onto the Registered Ports. The last
range is a recent addition by ICANN/IANA. They don't appear in RFC 1700 (now
However, the Registered Ports have been used by TCP/IP stacks for returning
connections. ie, you connect to a web site on port 80 and the connection to
you comes back to one of these ports, chosen at random. (The only exception
that I know to this is NTP, which uses UDP port 123 at both ends of the
connection). So simply blocking a port at a router may mean that legitimate
access is broken since the workstation has no way of knowing in advance that
any router has blocked this port. That gives a 64511/1 chance of breaking
access. For large organisations (or ISPs) with thousands of workstations,
blocking a Registered Port would result in numerous failures. This is
A long term solution would be to separate port usage in IPv6, if this has
not already been done. ie, to keep a range of numbers that are only ever
used for connecting to external machines and the IPv6 stacks stick to this
range. This will allow for better filtering by ISPs too, although someone
will probably still find a way of getting worms onto this range. It is sadly
too late now to fix the large number of "broken" IPv4 stacks.
Clearly therefore there will always be a demand for stateful firewalls,
whether on the workstation or as separate hardware. These can block
connections being created to the Registered Ports that do not already exist.
John Airey, BSc (Jt Hons), CNA, RHCE
Internet systems support officer, ITCSD, Royal National Institute of the
Bakewell Road, Peterborough PE2 6XU,
Tel.: +44 (0) 1733 375299 Fax: +44 (0) 1733 370848 John.Aireyrnib.org.uk
Nearly everything we believe is second hand. For example, less than 500
people have seen the Earth from space, yet the majority of people believe it
is round (OK pedants, an oblate sphere).
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