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Example 1:
(192.168.123.100-192.168.123.149) They are allowed to send mail (port 25), receive
mail (port 110), and browse Internet (port 80); port 53 (DNS) is necessary to resolve
the domain name.
(192.168.123.10-192.168.123.20) They can do everything (block nothing)
Others are all blocked.
Example 2: