Ip Packet Fields You Can Filter To Control Access - Cisco Catalyst 2960 series Configuration Manual

• Organize your access list so that more specific references in a network or subnet appear before more
general ones.
• Use the statement permit any any if you want to allow all other packets not already denied. Using the
statement permit any any in effect avoids denying all other packets with the implicit deny statement at
the end of an access list. Do not make your first access list entry permit any any because all traffic will
get through; no packets will reach the subsequent testing. In fact, once you specify permit any any, all
traffic not already denied will get through.
• Although all access lists end with an implicit deny statement, we recommend use of an explicit deny
statement (for example, deny ip any any). On most platforms, you can display the count of packets
denied by issuing the show access-listcommand, thus finding out more information about who your
access list is disallowing. Only packets denied by explicit deny statements are counted, which is why
the explicit deny statement will yield more complete data for you.
• While you are creating an access list or after it is created, you might want to delete an entry.
• You cannot delete an entry from a numbered access list; trying to do so will delete the entire access
• You can delete an entry from a named access list. Use the no permitor no deny command to delete
• In order to make the purpose of individual statements more scannable and easily understood at a glance,
you can write a helpful remark before or after any statement by using the remark command.
• If you want to deny access to a particular host or network and find out if someone from that network or
host is attempting to gain access, include the log keyword with the corresponding deny statement so
that the packets denied from that source are logged for you.
• This hint applies to the placement of your access list. When trying to save resources, remember that an
inbound access list applies the filter conditions before the routing table lookup. An outbound access list
applies the filter conditions after the routing table lookup.

IP Packet Fields You Can Filter to Control Access

You can use an extended access list to filter on any of the following fields in an IP packet. Source address
and destination address are the two most frequently specified fields on which to base an access list:
• Source address--Specifies a source address to control packets coming from certain networking devices
or hosts.
• Destination address--Specifies a destination address to control packets being sent to certain networking
devices or hosts.
• Protocol--Specifies an IP protocol indicated by the keyword eigrp, gre, icmp, igmp, ip, ipinip, nos,
ospf, tcp, or udp, or indicated by an integer in the range from 0 to 255 (representing an Internet protocol).
If you specify a transport layer protocol (icmp, igmp, tcp, or udp), the command has a specific syntax.
• Ports and non-contiguous ports--Specifies TCP or UDP ports by a port name or port number. The
• TCP flags--Specifies that packets match any flag or all flags set in TCP packets. Filtering on specific
list. If you need to delete an entry, you need to delete the entire access list and start over.
the appropriate entry.
port numbers can be noncontiguous port numbers. Port numbers can be useful to filter Telnet traffic
or HTTP traffic, for example.
TCP flags can help prevent false synchronization packets.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
Information About Access Control Lists
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