Why can't I use "talk" to talk with my friend on machine X?

Why can't I use "talk" to talk with my friend on machine X?

Unix has three common "talk" programs, none of which can talk with
any of the others. The "old" talk accounts for the first two types.
This version (often called otalk) did not take "endian" order into
account when talking to other machines. As a consequence, the Vax
version of otalk cannot talk with the Sun version of otalk.
These versions of talk use port 517.

Around 1987, most vendors (except Sun, who took 6 years longer than
any of their competitors) standardized on a new talk (often called
ntalk) which knows about network byte order. This talk works between
all machines that have it. This version of talk uses port 518.

There are now a few talk programs that speak both ntalk and one
version of otalk. The most common of these is called "ytalk".



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