Cisco Cisco Email Security Appliance C680 User Guide
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AsyncOS 9.1.2 for Cisco Email Security Appliances User Guide
A P P E N D I X
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Assigning Network and IP Addresses
This appendix describes general rules on networks and IP address assignments, and it presents some
strategies for connecting the Cisco appliance to your network.
strategies for connecting the Cisco appliance to your network.
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Ethernet Interfaces
For information about management and data ports on your appliance, see
Selecting IP Addresses and Netmasks
When you configure the network, the Cisco appliance must be able to uniquely select an interface to send
an outgoing packet. This requirement will drive some of the decisions regarding IP address and netmask
selection for the Ethernet interfaces. The rule is that only one interface can be on a single network (as
determined through the applications of netmasks to the IP addresses of the interfaces).
an outgoing packet. This requirement will drive some of the decisions regarding IP address and netmask
selection for the Ethernet interfaces. The rule is that only one interface can be on a single network (as
determined through the applications of netmasks to the IP addresses of the interfaces).
An IP address identifies a physical interface on any given network. A physical Ethernet interface can
have more than one IP address for which it accepts packets. An Ethernet interface that has more than one
IP address can send packets over that interface with any one of the IP addresses as the source address in
the packet. This property is used in implementing Virtual Gateway technology.
have more than one IP address for which it accepts packets. An Ethernet interface that has more than one
IP address can send packets over that interface with any one of the IP addresses as the source address in
the packet. This property is used in implementing Virtual Gateway technology.
The purpose of a netmask is to divide an IP address into a network address and a host address. The
network address can be thought of as the network part (the bits matching the netmask) of the IP address.
The host address is the remaining bits of the IP address. The number of bits in a four octet address that
are significant are sometimes expressed in CIDR (Classless Inter-Domain Routing) style. This is a slash
followed by the number of bits (1-32).
network address can be thought of as the network part (the bits matching the netmask) of the IP address.
The host address is the remaining bits of the IP address. The number of bits in a four octet address that
are significant are sometimes expressed in CIDR (Classless Inter-Domain Routing) style. This is a slash
followed by the number of bits (1-32).
A netmask can be expressed in this way by simply counting the ones in binary, so
255.255.255.0
becomes “
/24
” and
255.255.240.0
becomes “
/20
”.