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If I enter for example "block" as a firewall rule.

Does that mean it blocks the IPs to or does it mean something else ?

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up vote 7 down vote accepted

This is in CIDR notation (Classless Inter Domain Routing).

is the same as to

In reality though, you will only be able to use to for assigning to hosts.

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no. /20 is the netmask, not part of the ip address. /20 is the same as and refers to 4096 addresses, not 20.

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If that's a block rule on your firewall it means it will block traffic from the ip address range through, inbound or outbound depending on the rule's disposition (inbound or outbound traffic\service rule).

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