Ethernet Networking- P6

Ethernet Networking- P6:One of the biggest problems when discussing networking is knowing whereto start. The subject of computer networks is one of those areas for whichyou have to "know everything to do anything." Usually, the easiest way toease into the topic is to begin with some basic networking terminology andthen look at exactly what it means when we use the word Ethernet.
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Ethernet Networking- P6138 Routing Note: If you supply a firewall as a standalone appliance, you may want to turn the routers firewall off. More in Chapter 10. By default, most of todays small routers block packets from well-known ports. If you want to let them through, or want to let through traffic from specific Web applications such as games, then you will need to open the ports manually, as in Figure 6-10. You enter the ports you want to open in the Start and End boxes. (These make it easier to enter a range of ports.) If you have a Web server or FTP server with static IP addresss, you will need to open their ports, for example.Figure 6-10: Configuring a router to open specific ports Finally, you can usually configure Internet access policies (Figure 6-11), providing access controls for specific machines on your internal network.Adding Routers to an Ethernet 139 First, you create a list of workstations to be affected by the policy, as in Figure 6-12. Then you indicate when you want to deny or allow access. Notice also at the bottom of the access policy screen that you can block Web sites by URL or keyword. (It may not be as flexible as many stand- alone parental control applications, but its a start!) Figure 6-11: Configuring Internet access policies Note: You may have noticed that this router also has a screen for configuring wireless connections. Well look at that in Chapter 7.140 Routing Figure 6-12: Setting up a list of PCs for an Internet access policy Integrating Wire/ess TransmissionsIf you read the popular press, you would think that small networks werewireless, and nothing but wireless. The ostensible ease of setting up andusing a wireless network seems to be endlessly appealing. And there is noquestion that a wireless connection is convenient for connecting a comput-er such as a laptop that needs only occasional access to your network orthat changes its location frequently. However, there are major drawbacksto wireless networks~especially in terms of s e c u r i t y ~ t h a t should makeeven the smallest of small business users think twice.In this chapter well look at why the most common wireless networksarent truly Ethemet (and why they cant be). Well also talk about wirelessstandards and speeds, along with how wireless connections work. Alongthe way well explore the security issues that still plague todays wirelessconnections. 141142 Integrating Wireless TransmissionsWireless MAC Protocol versus EthernetMAC Protocol As you will remember, the Ether MAC protocol (CSMA/CD) relies on the ability of connected devices to detect the presence of a signal on the net- work wire. When a device detects a signal, it knows that the wire is in use and that it must wait to transmit. Wireless connections, however, cant use CDMA/CD. Why? Because wireless devices cant detect collisions. And why not? Because wireless transmissions are half duplex. With CSMA/CD, the transmitting device must send a flame and then imme- diately listen for a collision. But a wireless device cant send and listen at the same time. Therefore, if it transmits and a collision occurs, it has no way to detect that collision. CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) tries to minimize collisions. It works in the following way: 1. A device waiting to transmit checks to see if there is a carrier signal (access point is busy). 2. If the access point is not busy, it sends a jamming signal to alert other devices that it will be transmitting. 3. If there is a signal, the device waits a random amount of time and then checks the transmission channel again. 4. If the access point is still busy, the device doubles its wait time, and continues to do so until it can gain control of the tramission frequency. The randomness of the wait intervals and the increasing wait time mini- mize the collisions. Packets that are mangled by collisions wont generate TCP acknowledgment ...