Monday, 2 July 2012



1. Access lists are primarily used for two purposes:
1. Controlling traffic through a router, and
2. Controlling VTY access to a router’s VTY ports

2. IP access lists are a sequential list of permit and deny conditions that apply to IP addresses or upper-layer protocols. Access Control Lists are used in routers to identify and control traffic.

There are two types of IP access lists:

A. Standard IP Access Lists: These have the format,
access-list [number] [permit or deny] [source_address]

Keep in mind that:

1. Place standard access lists as near the destination as possible and extended access lists as close to the source as possible.
2. Access lists have an implicit deny at the end of them automatically. Because of this, an access list should have at least one permit statement in it; otherwise the access list will block all remaining traffic.
3. Access lists applied to interfaces default to outbound if no direction is specified.

B. Extended IP Access Lists: IP Extended Access lists have the format,
access-list {number}{permit or deny} {protocol} {source} {destination} {port}
With extended IP access lists, we can act on any of the following:
- Source address
- Destination address
- IP protocol (TCP, ICMP, UDP, etc.)
- Port information (WWW, DNS, FTP, etc.)

The permitted numbers for some important access-lists are:
1-99: IP standard access list
100-199: IP extended access list
800-899: IPX standard access list
900-999: IPX extended access list
1000-1099: IPX SAP access list
1100-1199: Extended 48-bit MAC address access list

3. Wild card masking:

Wild card masking is used to permit or deny a group of addresses. For example, if we have a source address and want all the hosts on the last octet to be considered, we use a wild card mask,

Special cases:

Host is same as with a wild card mask of, considers only specified IP.
Any is equivalent to saying with a wild card mask of This means none of the bits really matter. All IP addresses need to be considered for meeting the criteria.

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