computer science

Internet Connection Sharing

Internet Connection Sharing is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Internet Connection Sharing rather than just read about it. In short: Internet Connection Sharing (ICS) is a Windows service that enables one Internet-connected computer to share its Internet connection with other computers on a local area network (LAN). The computer that shares its Internet connection serves as a gateway device, meaning that all traffic between other computers and the Internet go through this computer.

Key takeaways

Internet Connection Sharing belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Internet Connection Sharing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Internet Connection Sharing from memory before moving on to harder problems.

Reference excerpt

Internet Connection Sharing (ICS) is a Windows service that enables one Internet-connected computer to share its Internet connection with other computers on a local area network (LAN). The computer that shares its Internet connection serves as a gateway device, meaning that all traffic between other computers and the Internet go through this computer. ICS provides Dynamic Host Configuration Protocol (DHCP) and network address translation (NAT) services for the LAN computers. ICS was a feature of Windows 98 SE and all versions of Windows released for personal computers thereafter.

Operation ICS routes TCP/IP packets from a small LAN to the Internet. ICS provides NAT services, mapping individual IP addresses of local computers to unused port numbers in the sharing computer. Because of the nature of the NAT, IP addresses on the local computer are not visible on the Internet. All packets leaving or entering the LAN are sent from or to the IP address of the external adapter on the ICS host computer. Typically, ICS can be used when there are several network interface cards installed on the host computer. In this case, ICS makes an Internet connection available on one network interface to be accessible to one other interface that is explicitly designated as the private network. ICS can also share dial-up (including PSTN, ISDN and ADSL connections), PPPoE and VPN connections. Starting with Windows XP, ICS is integrated with UPnP, allowing remote discovery and control of the ICS host. It also has a Quality of Service Packet Scheduler component. When an ICS client is on a relatively fast network and the ICS host is connected to the Internet through a slow link, Windows may incorrectly calculate the optimal TCP receive window size based on the speed of the link between the client and the ICS host, potentially affecting traffic from the sender adversely. The ICS QoS component sets the TCP receive window size to the same as it would be if the receiver were directly connected to the slow link. ICS also includes a local DNS resolver in Windows XP to provide name resolution for all network clients on the home network, including non-Windows-based network devices. When connected to a Windows domain, the computer can have a Group Policy to restrict the use of ICS, but when at home, ICS can be enabled.

Limitations The service is not customizable in terms of which addresses are used for the internal subnet, and contains no provisions for bandwidth limiting or other features. ICS was initially designed to connect only to Windows computers: computers on other operating systems required different steps to utilize ICS. On Windows XP, the server, by default, gets the IP address 192.168.0.1. (This default can be changed within the interface settings of the network adapter or in the Windows Registry.) It provides NAT services to the entire 192.168.0.x subnet, even if the address on the client was set manually, not by the DHCP server. Since Windows 7, the 192.168.137.x subnet has been used by default.

Alternatives Alternatives to ICS include hardware home routers and Wireless access points with integrated Internet access hardware, such as broadband over power lines, WiMAX or DSL modems.

Worked examples

Example 1 — a first encounter with Internet Connection Sharing

Start with the simplest possible case. Write down what Internet Connection Sharing claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Internet Connection Sharing before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Internet Connection Sharing ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Internet Connection Sharing

In research: Internet Connection Sharing appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Internet Connection Sharing in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Internet Connection Sharing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Routing software, Windows communication and services, so understanding it makes those chapters shorter.
In everyday life: Look for Internet Connection Sharing outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Internet Connection Sharing in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Internet Connection Sharing means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Internet Connection Sharing out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Internet Connection Sharing in simple terms?

Why does Internet Connection Sharing matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Internet Connection Sharing?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Internet Connection Sharing.