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Chapter 9

Intermediate TCP/IP/ Access Control Lists (ACLs)

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Objectives

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TCP Operation

The transport layer is responsible for the reliable transport of and regulation of data flow from source to destination.

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Synchronization or Three-Way Handshake

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Denial-of-Service Attacks

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Simple Windowing

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TCP Sequence and Acknowledgment Numbers

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Positive ACK

• Acknowledgement is a common step in the synchronization process which includes sliding windows and data sequencing.

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Protocol Graph: TCP/IP

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UDP Segment Format

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Port Numbers

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Telnet Port Numbers

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Reserved TCP and UDP Port Numbers

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Ports for Clients

• Whenever a client connects to a service on a server, a source and destination port must be specified.

• TCP and UDP segments contain fields for source and destination ports.

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Port Numbering and Well-Known Port Numbers

• Port numbers are divided into three different categories:

well-known ports

registered ports

dynamic or private ports

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Port Numbers and Socket

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Comparison of MAC addresses, IP addresses, and port numbers

• A good analogy can be made with a normal letter.

• The name on the envelope would be equivalent to a port number, the street address is the MAC, and the city and state is the IP address.

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Summary

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Access Control Lists (ACLs)

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Objectives

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What are ACLs?

• ACLs are lists of conditions used to test network traffic that tries to travel across a router interface. These lists tell the router what types of packets to accept or deny.

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How ACLs Work

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Protocols with ACLs Specified by Numbers

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Creating ACLs

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The Function of a Wildcard Mask

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Verifying ACLs

• There are many show commands that will verify the content and placement of ACLs on the router.

show ip interface

show access-lists

Show running-config

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Standard ACLs

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Extended ACLs

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Named ACLs

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Placing ACLs

• Standard ACLs should be placed close to the destination.

• Extended ACLs should be placed close to the source.

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Firewalls

A firewall is an architectural structure that exists between the user and the outside world to protect the internal network from intruders.

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Restricting Virtual Terminal Access

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Summary

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Question/Answer