[ih] Dotted decimal notation

[Jack Haverty]

IIRC, this convention was created over time by a coalescence of "rough
consensus and running code" as the early user programs (Telnet and FTP)
were being rewritten to use TCP instead of NCP, so it would have been
during the late 70s.   On the ARPANET, e.g., when a particular Telnet,
you would type "O <host>/<imp>", e.g., 1/6 to connect to MIT-DMS, host 1
on IMP 6, or "O 70" which was the equivalent.   Something new was needed
for specifying 32-bit IP addresses.

Dotted quad was one early approach, where the 4 numbers could be either
octal, if they had a leading zero, and otherwise decimal. 
A leading 0 indicated that the number was octal - also a common
convention in programming languages at the time.

The "dotted decimal" convention evolved from the "dotted quad", with the
difference being that the numbers in the "...decimal" form were of
course always decimal, regardless of the presence of a leading zero.  

I believe all of these forms were created as various people wrote user
programs.  The notation is really a design decision of the user
interface, converting typed IP addresses into the appropriate 32-bit
fields for the underlying TCP code.

Some people liked decimal numbers, others liked octal. 

One particularly irritating choice was pure decimal, i.e., a 32-bit
number represented in decimal (no dotted quad).   The early SRI TIU
(terminal concentrator) required the user to input decimal numbers,
which were annoyingly difficult to calculate.    E.g., 10.0.0.5, easily
recognized as Host 0 on ARPANET IMP 5, had to be typed in its 32-bit
decimal format when specifying what remote computer the user wanted to
access.  It was difficult to do such calculations in your head; I
remember pulling out a calculator to create the appropriate many-digit
decimal number.

Eventually the "dotted quad" notation reached rough consensus and many
host implementations of user apps (Telnet, FTP) permitted that form of
specifying a target host.

The "dotted decimal" convention eventually superceded the "dotted quad"
notation because the quad form was often confusing. 

E.g., "ISIF in dotted decimal is 010.002.000.052, or 10.2.0.52", where
leading zeroes are ignored.  But in dotted quad,
010.002.000.052 and 10.2.0.52 would not be equivalent.  010 would be
network 8 rather than 10, and 052 would be 42 instead of 52.

I don't remember who first produced dotted decimal though.   I think
you'd have to look at the applications programs of the time (FTP,
Telnet) to see what each used for its UI.

/Jack


On 12/28/20 12:55 PM, Brian E Carpenter via Internet-history wrote:
> Can anyone recall when and by whom the dotted decimal notation for IPv4
> addresses was invented? This text first appeared in RFC820 (January 1983):
>
>    One commonly used notation for internet host addresses divides the
>    32-bit address into four 8-bit fields and specifies the value of each
>    field as a decimal number with the fields separated by periods.  This
>    is called the "dotted decimal" notation.  For example, the internet
>    address of ISIF in dotted decimal is 010.002.000.052, or 10.2.0.52.
>
> The leading zeroes are not considered valid these days.
>
> Thanks 
>    Brian Carpenter