SMB
protocol
Set-top
box
SGML (Standard
Generalized Markup Language)
SNMP (Simple
Network Management Protocol)
Subnet
and Subnet mask
The SMB protocol (Server Message Block protocol) provides a method for client applications in a computer to read and write to files on and to request services from server programs in a computer network. SMB can be used over the Internet on top of its TCP/IP protocol or on top of other network protocols such as IPX and NetBEUI. Using the SMB protocol, an application (or the user of an application) can access files at a remote server as well as other resources, including printers, mailslots, and named pipes. Thus, a client application can read, create, and update files on the remote server. It can also communicate with any server program that is set up to receive an SMB client request.
Microsoft Windows for Workgroups, Windows 95, and Windows NT all include client and server SMB protocol support. For UNIX systems, a shareware program, Samba, is available. The SMB protocol originated at Microsoft and has gone through a number of developments. A given client and server may implement different sets of protocol variations which they negotiate before starting a session.
Microsoft has offered a public or open version of SMB for the Internet to the Internet Engineering Task Force (IETF). Called the Common Internet File System (CIFS), this new protocol provides more flexibility than existing Internet applications such as the File Transfer Protocol (FTP). CIFS is envisioned as a complement to the Internet's HTTP protocol for Web browsing.
A set-top box is a device that enables a television set to become a user interface to the Internet. A set-top box is really a specialized computer that can "talk to" the Internet - that is, it contains a Web browser (which is really a Hypertext Transfer Protocol client) and the Internet's main program, TCP/IP.
The service to which the set-top box is attached may be through a telephone line as, for example, with Web TV, or through a cable TV company like TCI.
SGML is a standard for how to specify a document markup language or tag set. The specification itself is called a document type definition (DTD). SGML is not in itself a document language, but a description of how to specify one.
SGML is based on the idea that documents have structural and other semantic elements that can be described without reference to how such elements should be displayed. The actual display of such a document may vary, depending on the output medium and style preferences. Some advantages of documents based on SGML are:
SGML is based somewhat on earlier generalized markup languages developed at IBM, including General Markup Language (GML) and ISIL..
You can find out more at SGML University (http://www.sgml.com).
A Web file with the suffix of ".shtml" (rather than the usual ".htm") indicates a file that includes some information that will be added "on the fly" by the server before it is sent to you. A typical use is to include a "Last modified" date at the bottom of the page.
This HTTP facility is referred to as a server-side include. (Although rarely done, the server administrator can identify some other file name suffix than ".shtml" as a server-side include file.) You can think of a server-side include as a limited form of Common Gateway Interface (CGI) application. In fact, the CGI is not used. The server simply searches the server-side include file for CGI environment variables, and inserts the variable information in the places in the file where the "include" statements have been inserted.)
When creating a Web site, a good idea is to ask your server administrator which environment variables can be used and whether the administrator can arrange to set the server up so that these can be handled. Your server administrator should usually be able to help you insert the necessary include statements in an HTML file.
SNMP is the protocol governing network management and the monitoring of network devices and their functions. It is not necessarily limited to TCP/IP networks. The details of SNMP are in these Internet Engineering Task Force (IETF) Requests for Comments (RFCs):
RFC 1089 - SNMP over Ethernet
RFC 1140 - IAB Official Protocol Standards
RFC 1147 - Tools for Monitoring and Debugging TCP/IP Internets
and Interconnected Devices
[superceded by RFC 1470]
RFC 1155 - Structure and Identification of Management Information
for TCP/IP based internets.
RFC 1156 (H)- Management Information Base Network Management
of TCP/IP based internets
RFC 1157 - A Simple Network Managment Protocol
RFC 1158 - Management Information Base Network Management
of TCP/IP based internets: MIB-II
RFC 1161 (H)- SNMP over OSI
RFC 1187 - Bulk Table Retrieval with the SNMP
RFC 1212 - Concise MIB Definitions
RFC 1213 - Management Information Base for Network Management
of TCP/IP-based internets: MIB-II
RFC 1215 (I)- A Convention for Defining Traps for use with
the SNMP
RFC 1224 - Techniques for Managing Asynchronously-Generated
Alerts
RFC 1270 (I)- SNMP Communication Services
RFC 1303 (I)- A Convention for Describing SNMP-based Agents
RFC 1470 (I)- A Network Management Tool Catalog
RFC 1298 - SNMP over IPX
RFC 1418 - SNMP over OSI
RFC 1419 - SNMP over IPX
SMTP vs. X.400
SMTP is a TCP/IP standard and X.400 is an OSI standard. Nowadays X.400 is increasingly run over TCP/IP thus enhancing its popularity. It is highly reliable, more fully featured than Internet Standards, although also more complex.
One reason for X.400s popularity is a sister standard, X.500, which provides white pages or directory services. Drawbacks – long and cumbersome mail addresses
Copies of the RFCs and a
Frequently-Asked Questions discussion on SNMP is available at
http://www.cis.ohio-state.edu/hypertext/faq/usenet/snmp-faq/part1/faq.htm
A subnet (short for "subnetwork") is an identifiably separate part of an organization's network. Typically, a subnet may represent all the machines at one geographic location, in one building, or on the same local area network (LAN). Having an organization's network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate subnetworks, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. It would also require that Internet routing tables on gateways outside the organization would need to know about and have to manage routing that could and should be handled within an organization.
The Internet is a collection of networks whose users communicate with each other. Each communication carries the address of the source and destination networks and the particular machine within the network associated with the user or host computer at each end. This address is called the IP address (Internet Protocol address). This 32-bit IP address has two parts: one part identifies the network (with the network number) and the other part identifies the specific machine or host within the network (with the host number). An organization can use some of the bits in the machine or host part of the address to identify a specific subnet. Effectively, the IP address then contains three parts: the network number, the subnet number, and the machine number.
The standard procedure for creating and identifying subnets is provided in Internet RFC 950.
The IP Address
The 32-bit IP address (we have a separate definition of it with more detail) is often depicted as a dot address (also called dotted quad notation) - that is, four groups of decimal digits (each group representing a quad or eight bits) separated by periods. Here's an example: 130.5.5.25
Each of the decimal digits represents a string of four binary digits. Thus, the above IP address really is this string of 0s and 1s:
10000010.000000101.00000101.00011001
As you can see, we inserted periods between each eight-digit sequence just as we did for the decimal version of the IP address. Obviously, the decimal version of the IP address is easier to read and that's the form most commonly used.
Some portion of the IP address represents the network number or address and some portion represents the local machine address (also known as the host number or address). IP addresses can be one of several classes, each determining how many bits represent the network number and how many represent the host number. The most common class used by large organizations (Class B) allows 16 bits for the network number and 16 for the host number. Using the above example, here's how the IP address is divided:
<--Network address--><--Host address-->
130.5 .
5.25
If you wanted to add subnetting to this address, then some portion (in this example, eight bits) of the host address could be used for a subnet address. Thus:
<--Network address--><--Subnet address--><--Host address-->
130.5 .
5 .
25
To simplify this explanation, we've divided the subnet into a neat eight bits but an organizationcould choose some other scheme using only part of the third quad or even part of the fourth quad.
The Subnet Mask
Once a packet has arrived at an organization's gateway or connection point with its unique network number, it can be routed within the organization's internal gateways using the subnet number as well. The way the router knows which bits to look at (and which not to look at) is looking at a subnet mask. A mask is simply a screen of numbers that tells you which numbers to look at underneath. In a binary mask, a "1" over a number says "Look at the number underneath"; a "0" says "Don't look." Using a mask saves the router of having to handle the entire 32 bit address; it can simply look at the bits selected by the mask.
Using the previous example (which is a very typical case), the combined network number and subnet number occupy 24 bits or three of the quads. The appropriate subnet mask carried along with the packet would be:
255.255.255.0
Or a string of all 1's for the first three quads (telling the router to look at these) and 0's for the host number (which the router doesn't need to look at). Subnet masking allows routers to move the packets on more quickly.
If you have the job of creating subnets for an organization (an activity called subnetting) and specifying subnet masks, your job may be simple or complicated depending on the size and complexity of your organization and other factors. Some of the "Additional Information" we list below may help.