3.2. La Campiña Segoviana
3.2.2. Bernardos y el Cerro del Castillo
A common practice in many organizations is to assign TCP/IP address space from RFC 1918. This practice serves many purposes:
■ It does not use registered IP space for wireless devices; which
typically do not include servers.
■ It enables the organization to subnet the address space
without any restrictions.
■ It allows for easy identification of WLAN traffic on the net-
work because it is not sharing address space with the wired network.
In many organizations, registered IP address space is a premium commodity. By using RFC 1918 address space, precious registered address space is not consumed by WLAN devices.
Subnetting can be taken to further extremes by using other masks that move the 1 further right from the original high-order bit placement.There are fixed values for subnet masks because you are restricted to having the high-order bits be consecutive 1’s.Table 1.2 shows the possible values for a subnet mask.
Table 1.2Subnet Mask Values
Class A Addresses Class B Addresses Class C Addresses
255.0.0.0 255.255.0.0 255.255.255.0 255.128.0.0 255.255.128.0 255.255.255.128 255.192.0.0 255.255.192.0 255.255.255.192 255.224.0.0 255.255.224.0 255.255.255.224 255.240.0.0 255.255.240.0 255.255.255.240 255.248.0.0 255.255.248.0 255.255.255.248 255.252.0.0 255.255.252.0 255.255.255.252 255.254.0.0 255.255.254.0 255.255.255.254
TCP
Transmission Control Protocol (TCP) is one of the protocols in the TCP/IP pro- tocol suite.TCP is a Layer four,Transport layer protocol that is responsible for establishing and maintaining reliable communications in a TCP/IP network.TCP also provides stream data transfer, efficient flow control, multiplexing, full-duplex communication, and reliability on the network at Layer four.
TCP is considered a reliable protocol in that every packet sent between nodes on a network is acknowledged before the next packet is sent.This might sound like high overhead to you—it is.TCP was developed to be used on networks that were less reliable than the networks we are used to working with in 2002.To accommodate for less reliable communication,TCP utilizes flow control and sequencing to ensure proper data flow.
As networks have stabilized,TCP has remained and is still in wide use today. On most IP networks,TCP is the most prevalent protocol because so many applications use TCP. Some of the more common applications include SMTP, FTP, HTTP, and Telnet.
UDP
UDP is one of the protocols in the TCP/IP protocol suite. UDP is a Layer four, Transport layer protocol that is used for applications that are not considered mis- sion-critical, require low overhead, or are streamlined for speed. Unlike TCP, UDP is not considered a reliable protocol. Rather UDP attempts to send packets on a “best-effort” service. It is connectionless and as such does not have the same overhead as TCP.
Some of the more common applications include SNMP, Network Time Protocol (NTP), Domain Name Service (DNS), and some Voice over IP (VoIP) implementations.
Summary
In this chapter, you have formed a solid foundation in basic networking to delve more deeply into WLANs.The chapter has reviewed some basics of networking including defining bus, star, ring, and mesh topologies.The most common deployment model for fully meshed networks would be in the WAN arena.
Two of the different access methods networks use include Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and deterministic access.
The OSI system model plays an important role in developing networking standards.The model is comprised of seven layers:
■ Physical layer ■ Data-link layer ■ Network layer ■ Transport layer ■ Session layer ■ Presentation layer ■ Application layer
The Physical layer of the OSI system model is responsible for defining the elec- trical and mechanical aspects of networking.The Data-link layer defines the proto- cols that control the Physical layer, determining such issues as how the medium is accessed and shared, how devices or stations on the medium are addressed and how data is framed before transmission on the medium.The Network layer is where protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP) are addressed, as well as where routing functions are performed.The Transport layer specifies a higher level of flow control, error detection, and correction.The Session layer is responsible for establishing, managing and terminating communication ses- sions between Presentation layer entities and the Transport layer.The Presentation layer is responsible for ensuring that data sent from the Application layer of one device is comprehensible by the Application layer of another device.The
Application layer is responsible for providing network services to applications such as e-mail, word processing, and file transfer that are not implicitly defined in the OSI system model.
After the OSI review, we applied the OSI system model to WLANs and dis- cussed the frame format the MAC layer uses in the 802.11 standard.We com- pared the familiar Ethernet frame format with a 802.11 frame to reveal
differences such as the Frame Control (FC) field as well as the four address fields. Moving up the OSI system model, we touched on how the other five layers of the OSI system model apply to WLANs.
Next, we covered some basics with TCP/IP including addressing and subnet- ting.There are five classes of TCP/IP addresses with classes A, B, and C being the predominant addresses. Class D addresses are reserved for multicast, and class E address space is reserved for future use by the IETF.Within the classes of addresses, you can more effectively utilize the address space by subnetting.We covered the seven basic subnet mask values that you will encounter in the real world as well.
Finally, we wrapped up the chapter with a quick review of TCP and UDP as part of the TCP/IP protocol suite.TCP is the reliable protocol with flow control and connection-oriented applications such as HTTP, FTP, and Telnet. UDP is considered “unreliable” and makes a best effort to deliver data. Many applications use UDP such as SNMP, NTP, and DNS.