Voice over internet protocol voip made easy

Cisco CCNA Certification Exam Tutorial: Access List Details You Must Know!

To pass the CCNA exam, you have to be able to write and troubleshoot access lists. As you climb the ladder toward the CCNP and CCIE, you'll see more and more uses for ACLs. Therefore, you had better know the basics!

The use of "host" and "any" confuses some newcomers to ACLs, so let's take a look at that first.

It is acceptable to configure a wildcard mask of all ones or all zeroes. A wildcard mask of 0.0.0.0 means the address specified in the ACL line must be matched exactly a wildcard mask of 255.255.255.255 means that all addresses will match the line.

Wildcard masks have the option of using the word host to represent a wildcard mask of 0.0.0.0. Consider a configuration where only packets from IP source 10.1.1.1 should be allowed and all other packets denied. The following ACLs both do that.

R3#conf t

R3(config)#access-list 6 permit 10.1.1.1 0.0.0.0

R3(config)#conf t

R3(config)#access-list 7 permit host 10.1.1.1

The keyword any can be used to represent a wildcard mask of 255.255.255.255.

R3(config)#access-list 15 permit any

Another often overlooked detail is the order of the lines in an ACL. Even in a two- or three-line ACL, the order of the lines in an ACL is vital.

Consider a situation where packets sourced from 172.18.18.0 /24 will be denied, but all others will be permitted. The following ACL would do that.

R3#conf t

R3(config)#access-list 15 deny 172.18.18.0 0.0.0.255

R3(config)#access-list 15 permit any

The previous example also illustrates the importance of configuring the ACL with the lines in the correct order to get the desired results. What would be the result if the lines were reversed?

R3#conf t

R3(config)#access-list 15 permit any

R3(config)#access-list 15 deny 172.18.18.0 0.0.0.255

If the lines were reversed, traffic from 172.18.18.0 /24 would be matched against the first line of the ACL. The first line is ?permit any", meaning all traffic is permitted. The traffic from 172.18.18.0/24 matches that line, the traffic is permitted, and the ACL stops running. The statement denying the traffic from 172.18.18.0 is never run.

The key to writing and troubleshoot access lists is to take just an extra moment to read it over and make sure it's going to do what you intend it to do. It's better to realize your mistake on paper instead of once the ACL's been applied to an interface!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com ), home of free CCNA and CCNP tutorials, and The Ultimate CCNA and CCNP Study Packages. For a copy of his FREE "How To Pass The CCNA" or "CCNP" ebook, visit the website and download copies! Our new RSS feed gives you free Cisco questions and tutorials daily!

MORE ARTICLES:

Time Management Tip: How to Stop Letting the Phone Put You Behind
Most people want and need a time management tip for effectively managing their use of the phone. Don?t be a slave to your phone learn how to master it and use it as the useful tool it can be. Unfortunately far too often the phone is your enemy sucking up tons of time with little benefit to show from all the time invested. The phone can provide a way to: get quick information without leaving your home or office, stay in touch and manage remote events and operations, leave a quick and useful message, and to save time by getting the information and help you need almost immediately. Let?s focus on its usefulness and identify ways to make it more useful to you than it may be now. Does your phone ring all day long or do you have people ...

Popular Daytime Television Show 'The View' Introduces New Video Phone To The Nation
On December 29, 2006 the popular television show, "The View" introduced the newest video phones and gave them as gifts to the audience. These phone allow a person see their caller and have other premium services at no extra cost.

Check By Phone
When you have been in business for a while, you know that success isn?t just about winning jobs or selling products. It?s also about getting paid. And the sooner the better.

Access Management Firm Traka Secures Second Queen's Award
A Buckinghamshire-based company that received the Queen's Award four years ago for its innovative technology, has now won the UK's most prestigious business award again, this time for its international sales.

What Is A Wireless Access Point?
What is a wireless access point The wireless access point is the place for your wireless devices to connect to the network

go2dental Exceeds 10 Million Member Mark and Continues to Enhance its Dental Service Offerings -- Insurer's Members Benefit From Access to Enhanc
go2dental exceeds 10 million member mark and continues to enhance its dental service offerings. Insurer's members benefit from access to enhanced services.

VoIP - A Laymans Look - Should You Or Shouldnt You?
"As business people we manufacture shin pads, or we distribute cat food, or we evangelize, but should we be considering VoIP? Will it make us more money, or save us time?" -Dennis SchooleySo what's all this VoIP hype anyway? I mean we all know that our voice can be delivered to the four corners of the globe over telephone lines. (Actually, I missed that part of second grade math where they taught us that a ball has corners, but everybody says it, so I'll accept it).

Website Promotion- Keys To Promoting Your Website
Website Promotion Basics- 7 Keys to Promoting Your Website Effectively

Cheap Mobile Phone Deals from Thephonenetwork
The mobile phone deals are a popular concept in the United Kingdom and each user of mobile phone use in one form or another. The mobile phone users have grown very fond of these mobile phone deals which have become very popular in the market today. The mobile phone deals are particularly suited to the lure potential customers to them. They are quite sprightly in nature and are very easy to get hold of. Lets have a look on mobile phone deals in detail.

Prader-Willi Syndrome Association (USA) Praises Congress for Postponing Vote on Bills Restricting Access to Human Growth Hormone
Abuse of Human Growth Hormone Generates Legislative Attention, Further Consideration