Ipv4 and ipv6 there are two kinds of ip addresses

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IPv4 and IPv6 There are two kinds of IP addresses because there are two versions of IP in use today. Version 4 is the most widely used. (Previous version numbers were used only in the Internet’s early experimental days, and you never see them on the Internet today.) IPv4 has a problem: its addresses are 32-bit numbers, meaning that there are only enough unique addresses for around 4 billion computers. That may sound like a lot, but it’s not enough, given how many computers and devices have Internet access and the rate at which new ones are coming online. We are already using ungainly hacks to enable multiple machines to share addresses, and limited IP address space is a big problem. In IPv6, an address is a 128-bit number, which provides sufficient address space for the foreseeable future, but there’s a problem. Old computers and routers don’t support IPv6. Computers can often be fixed with software upgrades—Windows XP can have IPv6 support installed (and it’s built into Windows Vista and later versions). But OS support is not the whole story—applications may also need to be updated. There’s a bigger problem for routers—a lot of them have the structure of IPv4 baked into their hardware, so they need to be replaced to get IPv6 support. This makes IPv6 seem like an unattractive choice—would you want your web server to have an address that will be inaccessible to anyone who hasn’t upgraded her network and Internet con- nection to IPv6? In fact, it’s not quite that bad, because there’s a special class of IPv6 addresses that are effectively equivalent to IPv4 addresses, so it’s possible to provide an IPv6-based server that’s accessible to IPv4 clients. But that means any public service you’re likely to want to use will be accessible from IPv4, so there’s not a whole lot of incentive for end users or corporate network administrators to throw out perfectly good IPv4 routers to up- grade to IPv6, and it means that phone companies don’t have many customers de- manding IPv6-capable DSL routers. Consequently, the transition to IPv6 is happening incredibly slowly. Nonetheless, the IPv4 address space problem isn’t going to go away, so you will need to write your software in a way that’s able to work with both IPv4 and IPv6 addresses if you want it to continue to work as IPv6 adoption gradually picks up. .NET tries to make this relatively easy in practice. Its IPAddress class can hold either kind of address. For most applications, client-side code doesn’t even need to be aware of which kind is in use. But occasionally, you’ll need to work with an IP address in its numeric form, at which point the distinction matters. While the Internet protocol uses numbers to identify machines, users are more familiar with names such as oreilly.com and . The Internet has a system called the Domain Name Service (DNS)—your Internet service provider gives you ac- 524 | Chapter 13: Networking
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cess to this as part of your connection—whose job is to convert these textual names
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