Chapter 9

False

False

True

True

True

C

A

B

D

C

D

A

B

A

C

B

D

C

A

C

B

A, D

A,B,D

B, C

A, C

segments or packets

hardware

host name

host

port forwarding

port triggering

ipconfig

Lightweight Directory Access Protocol (LDAP)

DHCP server

workgroup

Secure Shell (SSH)

Wi-Fi Protected Setup (WPS)

subnet mask

telnet

Network Address Translation (NAT)

Each client and server application installed on a computer listens at a predetermined address that uniquely identifies the application on the computer. This address is a number and is called a port number, port, or port address. For example, you can address a web server by entering into a browser address box an IP address followed by a colon and then the port number. These values are known as a socket. For example, an email server waiting to send email to a client listens at port 25, and a web server listens at port 80. Suppose a computer with an IP address of 136.60.30.5 is running both an email server and a web server application. If a client computer sends a request to 136.60.30.5:25, the email server that is listening at that port responds. On the other hand, if a request is sent to 136.60.30.5:80, the web server listening at port 80 responds.

The subnet mask used with IPv4 identifies which part of an IP address is the network portion and which part is the host portion. Using a subnet mask, a computer or other device can know if an IP address of another computer is on its network or another network. A subnet mask is a string of ones followed by a string of zeros. The ones in a subnet mask say, "On our network, this part of an IP address is the network part," and the group of zeros says, "On our network, this part of an IP address is the host part."

A computer using a private IP address on a private network can still access the Internet if a router or other device that stands between the network and the Internet is using NAT (Network Address Translation). NAT is a TCP/IP protocol that substitutes the public IP address of the router for the private IP address of the other computer when these computers need to communicate on the Internet.

A domain name identifies a network. Examples of domain names are the names that appear before the period in Microsoft.com, course.com, and mycompany.com. The letters after the period are called the top-level domain and tell you something about the domain. Examples are .com (commercial), .org (nonprofit), .gov (government), and .info (general use).

This process of associating a character-based name with an IP address is called name resolution. The DNS (Domain Name System or Domain Name Service) protocol is used by a DNS server to find an IP address for a computer when the fully qualified domain name is known. When Windows is trying to resolve a computer name to an IP address, it first looks in the DNS cache it holds in memory. If the computer name is not found in the hosts file, Windows then turns to a DNS server if it has the IP address of the server. When Windows queries the DNS server for a name resolution, it is called the DNS client.

In TCP/IP, the protocol that guarantees packet delivery is TCP (Transmission Control Protocol). TCP makes a connection, checks whether the data is received, and resends it if it is not. TCP is, therefore, called a connection-oriented protocol. TCP is used by applications such as web browsers and email. Guaranteed delivery takes longer and is used when it is important to know that the data reached its destination. For TCP to guarantee delivery, it uses protocols at the IP layer to establish a session between client and server to verify that communication has taken place. When a TCP packet reaches its destination, an acknowledgment is sent back to the source. If the source TCP does not receive the acknowledgment, it resends the data or passes an error message back to the higher-level application protocol.

UDP (User Datagram Protocol) does not guarantee delivery by first connecting and checking whether data is received; thus, UDP is called a connectionless protocol or best-effort protocol. UDP is used for broadcasting, such as streaming video or sound over the web, where guaranteed delivery is not as important as fast transmission. UDP is also used to monitor network traffic.

Lightweight Directory Access Protocol (LDAP) is used by various client applications when the application needs to query a database. For example, an email client on a corporate network might query a database that contains the email addresses for all employees. Another example is when an application looks for a printer by querying a database of printers supported by an organization on the corporate network or Internet. Data sent and received using the LDAP protocol is not encrypted; therefore, an encryption layer is sometimes added to LDAP transmissions.

A security key is required. This is the most common method of securing a wireless network. A network that uses a security key encrypts data on the network using an encryption standard. You learn about these standards later in the chapter. The SSID is not broadcast. The wireless device might not be broadcasting its name, which is called the Service Set Identifier (SSID). If the SSID is not broadcasting, the name of the wireless network will appear as Unnamed or Unknown Network. When you select this network, you are given the opportunity to enter the name. If you don't enter the name correctly, you will not be able to connect. Only computers with registered MAC addresses are allowed to connect. If MAC address filtering is used, you must give the network administrator the MAC address of your wireless adapter. This address is entered into a table of acceptable MAC addresses kept by the wireless access point.

Function 1: As a router, it stands between the ISP network and the local network, routing traffic between the two networks. Function 2: As a switch, it manages several network ports that can be connected to wired computers or to a switch that provides more ports for more computers. Function 3: As a DHCP server, all computers can receive their IP address from this server. Function 4: As a wireless access point, a wireless computer can connect to the network. This wireless connection can be secured using wireless security features. Function 5: As a firewall, it blocks unwanted traffic initiated from the Internet and provides Network Address Translation (NAT) so that computers on the LAN can use private or link local IP addresses. Another firewall feature is to restrict Internet access for computers behind the firewall. Restrictions can apply to days of the week, time of day, keywords used, or certain web sites. Function 6: As an FTP server, you can connect an external hard drive to the router, and the FTP firmware on the router can be used to share files with network users.