Networking is the process of connecting computing devices, peripherals and terminals together through a system that uses wiring, cabling or radio waves that enable their users to communicate, share information and interact over distances. Often associated are issues regarding operating systems, hardware and equipment, cloud and virtual networking, protocols, architecture, storage and management.

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AKA "why can't I reach my server on its external IP from an internal IP?"

As a Networking ZA/PE, I see a rather significant number of NAT-related (network address translation) questions whose problems can all be attributed to a single issue.  

Typically the scenario involves an internal network that is connected to the internet via some sort of NAT device. The internal network contains a server. The server hosts some arbitrary number of services. No internal client can connect to these services via an external IP that has been forwarded (via some sort of DNAT) to the server. This issue can even affect client processes on the server itself if those processes attempt to connect to the server via its externally forwarded IP.

To resolve this problem, an understanding of the underlying cause is helpful.

A NAT device exists with an external IP of It connects an internal network with the range 10.1.2.x to the internet via SNAT with an internal IP of A server exists on the internal network at IP All appropriate inbound connections to are forwarded (via DNAT) to A client with IP attempts to access a service on the server via the externally forwarded IP address of (typically, the client obtains the external IP through its name resolution/resolver engine which often includes configured DNS servers and a local hosts file). Here is the communication breakdown:

The client ( sends a …
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by:Eric AKA Netminder

Congratulations! Your article has been published, and is also set as a Community Pick, which includes bonus points.


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Want to improve the strength and reputation of your reviews on Set up a Real Name attribution.  Here’s how describes their Real Name attribution program:

A Real Name attribution is a signature based on the name entered by the author as the cardholder name on his or her credit card, i.e. the author represents this name as his/her identity in the "real world."

(It) establishes credibility much as reputations built over time in the community… Real Name attribution is just one input into a function that much more strongly weights "helpful" votes when votes are available.
Here’s how to sign up for a Real Name attribution.

If you don’t already have an account, sign up for one by clicking “Start here” at the top of the home page. Add a credit card to your account.

Once you have a registered account with a credit card, it’s time to set up your profile.

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Scroll to the bottom box labeled “Personalization” and click “Your Public Profile” under Community.
Click the “Create Profile” button. Add personal details like location and your webpage, a photo if you wish, but it is not necessary for your Real Name post. You may add a photo if you like, though it is not necessary.
Once you have finished setting up your profile you will be redirected to your public profile page. …


This article is aimed at someone who has never used IRC before. It covers the very basics of setting up and configuring a client and a little bit about starting a channel and the basics of being a channel operator. What it doesn't do is review or promote any of the various IRC clients available. Nor does it cover power users; however, suitable reference material is suggested at the end for eager readers. Also note, there are many IRC clients available and all are different. The details given in this article are completely generic, providing commands that should operate on all IRC clients.

So, firstly, what is IRC and why would you want to use it? Well, put simply IRC is the daddy of the various Instant Messaging (IM) services we use today. It's been around for almost as long as The Internet. The main difference between IM and IRC is that IM is mainly focused on one to one conversations, like a telephone call, whereas IRC is mainly focused on group conversation, just like a meeting. The main benefit of IRC over IM is the fact it is collaborative. This makes it ideal for companies to implement as a communications medium for teams that may be located on different sites. Each team can join their own specific channel. If teams need to discuss sensitive information these channels can be password protected so no one else can join without …
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Thanks Matt.
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Expert Comment

Voted yes.
There are numerous misunderstandings of the Ingress and Egress concepts when related to different OSI layers, so here is a brief overview:

There is no big philosophy when one keeps in mind that Ingress/Egress-terms were originally explaining OSI L2 features. So they are always switch port related. First we had "dumb" L2 switches with only physical ports. Then a frame - mind NOT a packet - from a PC1 to the switch port 1 is ingress and the same frame from 24 to PC2 is egress. To summarize as a definition on L2 ports: ingress is incoming from an adjacent node, egress outgoing to an adjacent node.

This concept was later needed to explain OSI L2 enhancements like VLAN and QoS where different tags were applied to the frame header and a decision had to be made from the switch, where exactly to add or strip them down.  So for example for a “client” switch port (called under Cisco "switchport mode access") belonging to a certain VLAN this header information had to be erased before egressing, whereas for a VLAN trunk port (i.e., switchport mode trunk) this header information had to be preserved by the egressing process.

Later on the terms were applied on L3-enhanced switches which brought some troubles since there we have L3 packets (this means with additional IP header) that are being routed and not switched. There physical ports and VLAN-ports mingled the straight understanding but the logic behind stayed the same – a bridged frame that has to cross-over VLANs is ingressing…

Expert Comment

Great information, thanks!

Expert Comment

The Internet has grown at a very rapid rate, and it only gets better by the day. Every device that’s connected to the Internet (be it some high end Web Server, VoIP Switches, your PC, that new sleek looking iPhone you've fallen in love with or any other device you may imagine) needs to have some unique identity. Without an identity the device cannot communicate or receive any communication. Think of it as your house address, or perhaps your phone number – it's unique and your colleagues and peers use it to reach to you. In Internet terms, this unique address is referred to as the IP Address.

The current IP Addressing system allows to have roughly about 3.3 billion unique addresses. However, with the rapid growth of the Internet and the devices connecting to it, there are simply not enough addresses available to uniquely allocate to all. Now, that’s a problem!  Nevertheless, every problem has a good, or sometimes, not so good solution. So, the solution for the scarce IP Address problem? – NAT.

NAT is a mechanism of taking a unique IP address and sharing it with a pool of devices.  You may despair, “That's like sharing my house address with someone else!”, or perhaps, “Thats like sharing my phone number with someone else!”, well, in a sense, it’s something like that, but its not that bad. Imagine your work place, not everyone has a unique/direct desk-phone number. You’ve got the primary phone number and employees have extensions. Your extension could be 1001, and your …

Networking is an area of computing where you must be thorough with what you implement. If not configured properly, it causes severe headaches in operations. This may lead to reconfiguring the network afresh.

Virtualization is a concept in which you trick your Operating System to see another computer (or device) connected to it. A subtle example would be the Optical Drives that your computer sees when you install Daemon Tools or Virtual Clone Drive software. Virtual Machines are just an extension of that technology relating to a Computer as a whole rather than a single device.

Basis of this article:

In this article, I will try to explain the implementation of Static Routes using a Virtual Machine and Windows XP.

Before we proceed, I would like you to have a thorough view of this article at Experts Exchange which explains “Static Routes” and “Default Gateways” by keith_alabaster in the best possible manner. My article will just illustrate how we could set it up in a practical environment.

Requirements on part of the reader:

Following will be needed to implement the contents of this article in a test environment-

  1.   Basic understanding of IP addresses and their behavior in a local area network
  2.   Installation …
I have seen a number of questions on this site asking about what hardware / simulation software is the best for building a CISCO lab for training, So here's a quick introduction to some of the most common used, along with strengths and weaknesses.

Below are the 4 common solutions you will see mentioned.

1: CISCO Packet Tracer 5.0 and above
2: Real Hardware
3: GNS3 (
4: other simulators such as Boson net sim

Starting with CISCO packet tracer, this is an ideal starting place for people new to Cisco. It has a great drag and drop interface, switches and routers can be configured with a point and click interface, as well as the command line. It also has a brilliant simulation mode where you can watch the packets travelling across  the devices. clicking on them at any point to see how the devices are dealing with them at the different levels of the OSI. It is a great way to see the fundamentals of the network taking place.
However there is one big problem with it... It is not a free download.. In fact it is quite hard to get hold of, but if you have a Cisco partner who can get hold of it for you, or you are part of the Cisco academy. This would be my number one choice for starting out in networks.

The second Solution is The real hardware . This is going to be the most expensive choice, but there really is nothing better than configuring the real thing. No matter how much I use simulations, the real thing still feels the …

Expert Comment

I use packet tracer to refresh my learning from cisco labs and it is decent.  Nothing beats the actual hardware as you mentioned.  Thanks for the good article.
Common Types Of DNS Records
With a basic knowledge of DNS, you know that different types of records are required to identify hosts and different types of services and attributes related to the domain. The most common types of DNS records used in most domains and their most common uses are described below.

A - Host - Usually used to associate a hosts name with an IPv4 address. For IPv6 see AAAA record later in this article.
MX - Mail Exchange - Used to identify mail servers that perform mail services for the domain.  
NS - Name Server - Identifies an authoritative DNS server for a particular zone.
PTR - Maps IP addresses to hosts for use in a reverse DNS lookup.
SRV - Service Locator - A general service record. Used generically instead of creating protocol specific records such as MX.

More Record Types
AAAA - IPv6 Host - Same as an A Record except it returns an IPv6 address.
CNAME - Canonical Name - Used to create an alias to reference one host by multiple names.
DNAME - Delegation Name - delegates an entire portion of a DNS tree under a new name not to be confused with CNAME which is for an individual name.
LOC - Location - Specifies a geographical location of a domain
SOA - Start of Authority - Contains authoritative information about a domain including: serial number which other servers for the zone can use to identify changes; primary name server; refresh to identify …
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Author Comment

Well its certainly not a required record type unless you have applications that support this. If you would like more input, I would suggest opening a question becuase maybe more experts might have an opinion or other information to share about this.

Without going into too much detail, here are a few uses I can think of
-If I were a search engine, I could use this information to lookup other nearby businesses to offer a an alternative or supplemental information.
-Allowing a company to dynamically view a worldwide IT infrastructure on a map(Generically of course).
-Advertising companies can use this data to serve ads more relevant to the host site
-With reverse DNS I might be able to more accurately geolocate my websites viewers on a map
-Applications could be extended to GPS, wifi, iPhone, etc.

I'm sure there are ways to do this alreayd without LOC data but its just an option.
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Expert Comment

Hopefully this comes across as constructive feedback because I'm in a rush :)

The article is too basic for anyone who knows anything at all about DNS and too basic for anyone who wants to learn anything about DNS and they would be best served looking elsewhere for information (Wikipedia, Google, etc).

CNAME: You should to explain this better. You can create mutliple A records to reference a host by many names as well.
DNAME: You should either elaborate on or provide working examples because anyone who has no experience with DNAME's will be no better off after reading this.
LOC: Why? I would not describe this as a common DNS record.

Maybe link off to a Wikipedia article or something which explains these and more types in detail if you're not going to actually write full article on DNS types.
DNS stands for Domain Naming System. DNS is a very complicated protocol and worldwide distributed database that translates our easy to remember words into actual addresses that computers can use to communicate. This article is not meant to go into all of the details of DNS, only give a high level overview.

Why does DNS exist?
In order to reach a server over the internet, an ip address is needed to contact the server. Instead of trying to remember addresses in number form such as, people more easily remember words such as

Who is responsible for DNS?
In short, DNS is owned by the world. No single organization is responsible for all of the DNS infrastructure.

How does it work?
Domain names consist of a top level domain, abbreviated by TLD (.com, .org, .net, .edu, etc.), a second level domain, and can contain one or more subdomains or hosts up to 127 levels.

1. The clients resolver sends the request to its assigned DNS server.

2. When a request is submitted to a DNS server, it goes through the following process.
a. Return the IP address for the domain if it knows it either by caching or if it is authoritative for the domain. If the server is authoritative for the domain and cannot find an address that matches the query, an authoritative answer is sent back to the client that the record does not exist.
b. If the …
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Author Comment

Any specific suggestions? I wouldn't mind editing.

Expert Comment

I would like to understand DNS enough to troubleshoot on a domain in a multi site network that is using VPN's and has local sites with their own subsets, file, print, SQL servers, internal / external DNS , using a hosts file etc, . I agree with the comment above its way too basic.






Networking is the process of connecting computing devices, peripherals and terminals together through a system that uses wiring, cabling or radio waves that enable their users to communicate, share information and interact over distances. Often associated are issues regarding operating systems, hardware and equipment, cloud and virtual networking, protocols, architecture, storage and management.