6.5 TCP/IP Networking & Configuration
Remote Desktop Protocol (RDP)
*Default ports:* 3389 *RDP* allows you to view and use the graphical desktop of a remote computer system as if you were sitting in front of it.
Dynamic Host Configuration Protocol (DHCP)
*Default ports:* 67 & 68 *DHCP* is used to dynamically assign IP addressing information to network hosts when they come online. The client system, when it connects to the network, broadcasts a DHCPDISCOVER message on the network, looking for a DHCP server. The DHCP server responds with a DHCPOFFER message containing proposed IP addressing configuration information. The client then responds with a DHCPREQUEST message to the DHCP server indicating it wants to use the proposed configuration. The DHCP server makes the assignment with a DHCPACK message.
Post Office Protocol 3 (POP3)
*Default port:* 110 *POP3* is part of the IP protocol suite and used to retrieve email from a remote server to a local client over an IP connection. With POP3, email messages are downloaded to the client.
Internet Message Access Protocol (IMAP)
*Default port:* 143 *IMAP* is an email retrieval protocol designed to enable users to access their email from various locations without the need to transfer messages or files back and forth between computers. Messages remain on the remote mail server and are not automatically downloaded to a client system.
Simple Network Management Protocol (SNMP)
*Default port:* 161 & 162 *SNMP* is used to monitor and manage network devices. SNMP agents can be installed on network devices such as PCs, switches, and routers. These agents send data to an SNMP manager application running on an administrative workstation, which aggregates the information and displays an overview of the current network status. Thresholds can be configured which trigger alerts if exceeded.
Simple Mail Transfer Protocol (SMTP)
*Default port:* 25 *SMTP* is used to route electronic mail through the internetwork. Email applications provide the interface to communicate with SMTP or mail servers.
Automatic Private IP Addressing (APIPA)
If a Windows client is configured to receive an IP address from a DHCP server, but cannot contact a DHCP server, it will automatically assign itself an IP address within the following range: *169.254.0.1 to 169.254.255.254 with the subnet mask 255.255.0.0* *With APIPA:* -The host is configured to obtain IP information from a DHCP server (this is the default configuration). -If a DHCP server cannot be contacted, the host uses APIPA to assign itself an IP address. -The host only configures the IP address and mask. It does not assign itself the default gateway and DNS server addresses. For this reason, APIPA can only be used on a single subnet. You can use APIPA instead of a DHCP server, but it should only be used on a small network that does not need default gateway or DNS server settings configured. *APIPA is the term used by Microsoft Windows. On other systems (e.g., Linux), this functionality is known as link-local addressing.*
Dynamic Host Configuration Protocol (DHCP)
On a network configured with DHCP, IP hosts contact a DHCP server for IP addressing and other configuration information. This ensures error-free host configuration. *With DHCP:* -The host uses broadcast messages to locate a DHCP server when it boots. -The DHCP server assigns IP address and masks values to the host (called an address lease). -The DHCP server ensures that no two hosts are assigned the same IP address. -You can configure the DHCP server to deliver default gateway, DNS server, and other configuration information to hosts. *DHCP requires a special DHCP server, which is built-in to most SOHO routers.*
DNS server address
The DNS server address identifies the DNS server that is used to resolve host names to IP addresses.
IP address
The IP address identifies both the logical host and the logical network addresses. -Each host on the entire network must have a unique IP address. -Two devices on the same subnet must have IP addresses with the same network portion of the address. -Two devices on the same subnet must have unique host portions of the IP address. -Do not use the first or the last host address on a subnet address range.
Default gateway
The default gateway identifies the router to which communications for remote networks are sent. The default gateway address is the IP address of the router interface on the same subnet as the local host. Without a default gateway set, most clients will be unable to communicate with hosts outside of the local subnet.
Hostname
The hostname identifies the logical name of the local system.
Subnet Mask
The subnet mask identifies which portion of the IP address is the network address and which portion is the host address. Two devices on the same subnet must be configured with the same subnet mask.
File Transfer Protocol (FTP)
*Default port:* 21 *FTP* provides a generic method of transferring files. It can include file security through usernames and passwords, and it allows file transfer between dissimilar computer systems. FTP can transfer both binary and text files, including HTML, to another host. FTP URLs are preceded by ftp://followed by the DNS name of the FTP server. To log into an FTP server, use: ftp://username@servername.
Remote Terminal Emulation (Telnet)
*Default port:* 23 *Telnet* allows an attached computer to act as a dumb terminal, with data processing taking place on the IP host computer. It is still widely used to provide connectivity between dissimilar systems. Telnet can also be used to test a service by the use of HTTP commands. You should avoid using Telnet as it transmits all data (e.g., usernames and passwords) clear text.
Lightweight Directory Access Protocol (LDAP)
*Default port:* 389 & 636 *LDAP* is a protocol used to access information about network resources stored by a directory service, such as Active Directory or eDirectory. LDAP uses port 389 for clear text transmissions and port 636 for secure transmissions.
Service Location Protocol (SLP)
*Default port:* 427 *SLP* is a protocol that is able to organize and locate various network devices and services, such as printers, shared disk drives, directories, etc.
Hypertext Transfer Protocol over Secure Socket Layer or HTTP over SSL (HTTPS)
*Default port:* 443 *HTTPS* is a secure form of HTTP that uses SSL as a sublayer for security. SSL secures messages being transmitted on the Internet. It uses RSA for authentication and encryption. Web browsers use SSL (Secure Sockets Layer) to ensure safe Web transactions. URLs that begin with https://trigger your Web browser to use SSL.
Server Message Block (SMB)
*Default port:* 445 *SMB* enables the sharing of folders and printers on the network. Using SMB, remote users can access files in a shared folder on a server or workstation. Likewise, a remote user can send print jobs to a shared printer. SMB is also known as Common Internet File System (CIFS). *SMB running directly over TCP uses port 445.* *SMB running on "NetBIOS over TCP/IP" uses UDP ports 137 & 138 and TCP ports 137 & 139.*
Domain Name System (DNS)
*Default port:* 53 *DNS* is a system that is distributed throughout the internetwork to provide address/name resolution. For example, the name www.mydomain.com would be identified with a specific IP address.
Apple Filing Protocol (AFP)
*Default port:* 548 *AFP* is the protocol used by systems running Mac OS X or newer to support file sharing on the network. AFP replaced AppleTalk and has gone through several revisions.
Hypertext Transfer Protocol (HTTP)
*Default port:* 80 *HTTP* is used by web browsers and web servers to exchange files (such as web pages) through the World Wide Web and intranets. HTTP can be described as an information requesting and responding protocol. It is typically used to request and send Web documents but is also used as the protocol for communication between agents using different IP protocols.
Secure Shell (SSH) & Secure FTP (SFTP) protocols
*Default ports:* 22 *SSH* allows for secure interactive control of remote systems. SSH is a much more secure alternative to Telnet. *SFTP* addresses one of the key weaknesses of FTP; namely, FTP doesn't use encryption. All data, including usernames and passwords, is sent a clear text. SFTP provides the same functionality as FTP but secures the data transmissions using the SSH protocol.
NetBIOS
*NetBIOS* is the term used to describe the combination of two protocols: NetBEUI and NetBIOS. NetBIOS was used in early Windows networks. Because NetBIOS is a non-routable protocol, it was often combined with IP to enable internetwork communications. *NetBIOS over TCP/IP, or NetBT* is used to allow older computers and applications that rely on NetBIOS to communicate on a TCP/IP network.
Network Protocol
A rule that identifies some aspect of how computers communicate on a network. For two computers to communicate, they must be using the same protocols. Protocols are grouped into protocol suites, or sets of related protocols, that are meant to be used together.
TCP/IP
*TCP/IP* is the protocol suite used on the Internet and on most networks. Nearly all computers today use TCP/IP for communication. The Internet Protocol (IP) is a key component of the TCP/IP protocol suite. The IP protocol is responsible for determining how to deliver data from the sending host to the destination host. However, it does not provide a mechanism for segmenting and sequencing packets in a communication. To accomplish this, IP is used in conjunction with another transport protocol: *Transmission Control Protocol (TCP)* - TCP is a connection-oriented protocol. To ensure reliable delivery of data, TCP requires the recipient of a network transmission to send an acknowledgement of each and every IP packet it receives to the sender. Packets that don't make it are retransmitted. This ensures that the data is delivered reliably. *User Datagram Protocol (UDP)* - UDP is a connectionless protocol. Unlike TCP, UDP does not require acknowledgments. One of the key drawbacks of using TCP is the fact that its reliability introduces latency. For small data transmissions, such as sending an email, moderate latency is not a problem. However, for large data transmissions, such as video or audio streaming, the latency associated with TCP is unacceptable. By using UDP instead, the latency of the transmission is significantly reduced, with the assumption that an occasional lost packet won't be detrimental.
You should know the following facts about IP address assignments:
-By default, all Windows computers try to use DHCP for TCP/IP configuration information. -If your computer has used APIPA because the DHCP server is unavailable, it will have an IP address in the 169.254.x.x range. The workstation will be able to communicate with other hosts that have addresses in this range, but will not be able to communicate with hosts on other networks. -When you configure a static IP address, you disable DHCP and APIPA. -If you use a DHCP server for IP address configuration, you can still manually configure DNS server addresses. -By default, a Windows workstation will use the hosts file to resolve hostnames into IP addresses first. If a match can't be found for the hostname to be resolved in the hosts file, the workstation will contact the DNS server it has been configured to use.
Be aware of the following facts regarding protocol suite support and features:
-Virtually all operating systems today provide native (built-in) support for TCP/IP. -Most older versions of some operating systems used a different protocol as the default protocol suite. For example, older NetWare servers used IPX/SPX, while older Mac OS systems used AppleTalk. -Older operating systems without native TCP/IP support enabled IP communications by either installing the protocol stack or through a process is known as *encapsulation or tunneling.* With this process, non-IP packets are re-packaged as IP packets at the sending device. The receiving device strips off the IP headers to reveal the original packets.
Alternate IP configuration
With an alternate IP configuration, the system attempts to use DHCP for TCP/IP configuration information. If a DHCP server cannot be contacted, the static configuration values are used. When you configure an alternate IP address, APIPA is no longer used. *Use an alternate configuration:* -If you have a computer (e.g., a laptop) that connects to two networks (i.e., one with a DHCP server and another without a DHCP server). -If you want to provide values to properly configure the computer in case the DHCP server is unavailable.
Static addressing
With static addressing, you manually assign all configuration values. Static addressing is prone to error and should only be used under the following conditions: -The network has a small number of hosts. -The network will not change or grow. -You have some hosts that must have the same address each time.