The most popular services are WWW and eMail, based on protcols called HTTP, SMTP and POP3. Other well known services include FTP, Usenet (news - protocol NNTP), Telnet and IRC. If you don't understand what words like "service" or "protocol" mean, just picture all this stuff as alternatives or enhancments to well known electronic communication services like televison, voice phone or fax.
There are also other experimental services, like Internet Phone, See-You-See-Me, PointCast network, ICQ or Internet Radio, which are not generaly understood as Internet standards. Some of the Internet protocols, like Gopher or Finger are remnants from an earlier chapter in the history of the Internet. It is very interesting and if you are interested in it, try to get a book about the Internet.
Most important among TCP/IP protocols are TCP (transmission control protocol), IP (internet protocol) and UDP (user datagram protocol). Pieces of data transported by TCP/IP are called packets.
Advanced operating systems (like Unix) implement TCP/IP as a basic system service (on the same level as other I/O operations); in small systems like DOS, TCP/IP must be integrated into the application. This is why configuring Arachne is so confusing: you are configuring both TCP/IP and the browser (and if you are using a modem, then a PPP dialer also) all in one program! With Arachne, you are provided with a complete network solution, which together with the operating system fits onto one floppy and can be booted from it.
TCP/IP was designed for permanently connected computers; if you want to connect using a phone line and modem, you will need to establish a SLIP or PPP protocol between your Internet provider and your computer to transfer TCP/IP packets to your computer.
Arachne is a WWW client - a program which can request, download and view HTML pages. The WWW uses a protocol called HTTP.
The only mass communication technology that is as easy to use as the WWW is probably television. Users of the World Wide Web are expected only to be able to read and to know how to use a mouse to select hypertext links, nothing more.
Electronic mail is simply an exchange of text messages (which can contain attached encoded files) between Internet users. Arachne can currently send and receive simple eMail messages, using protocols called SMTP (sending eMails) and POP3 (downloading eMail from mailbox). Every user of Internet mail is assigned a unique address of format username@domain. Your username does not have to be identical to your login name. The domain is usually (but not always) the name of your provider's mail server.
Having a personal eMail address is probably as common today as having a personal telephone number.
The URL syntax is protocol://[username[:password]@]hostname[:port][/URI]
URI is a shortcut for Uniform Resource Identifier, and it is in fact very often just a filename of a document or program located on the WWW server.
OpenDOS is the latest member of Digital Research's CP/M development line: CP/M ... DR-DOS ... Novell DOS ... Caldera (DR)OpenDOS.
But you are likely to have a 386 or better processor which can be switched to "protected mode" (also called "32bit" mode) to access all the memory. From DOS, you can do this, for example, by using a DPMI driver. You are probably asking why Arachne is a real mode application, running only in the first 640 kB of your PC's memory. The reason is that the golden era of DOS ocurred on real mode computers, and most development tools, libraries, and helper applications feel best in real or "V8086" (also called "Virtual") mode. Developing applications for protected mode DOS is a much more complex task.
Arachne, as a real mode application, is a very small program and fits onto a single floppy: compare this to protected mode applications, written in object-oriented languages. They are ten times larger and take forever to download from the Internet. Arachne was developed mainly for PCs with a 80286-, 80386-, or 80486-compatible CPU and with only several MB of RAM, where real mode DOS applications perform much better than Windows applications. On Pentium P5/P6 (trademark of Intel), AMD K5/K6 or various 80586 and 80686 machines with more than 8 MB of memory, real mode applications are not able to use the full potential of the system. However, you can use an extended memory manager like EMM386 or QEMM to optimize a "virtual DOS machine" on your 386 or better computer. This is very efficient and actually much faster than the operating systems which are expected to run on these machines. Using DOS on Pentium class computers probably can give you more "real-time" feel then using any other OS.
Check your system manual or configuration utility for information on how to configure a RAM disk on your PC.