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One of these is called Dynamic Host Configuration Protocol (DHCP); it’s the service that recognizes computers and other devices
A second basic service that is provided by one of the servers on your network is called Domain Name System (DNS). This service is what enables people to use network names instead of the actual addresses that are handed out by DHCP.
You should never install anything less than Category 5 cable. And if at all possible, invest in Category 5e (the e stands for enhanced) or even Category 6
When you use UTP cable, you must be careful not to route cable close to fluorescent light fixtures, air conditioners, or electric motors (such as automatic door motors or elevator motors).
Note that you should not chain more than three switches together. As explained in Chapter 2 of this minibook, you can get around this rule by using stackable switches. Stackable switches have a special type of cable connector that connects two or more switches in a way that lets them function as if they were a single switch. Stackable switches are a must for large networks.
To set up two or more access points for roaming, you must carefully place the WAPs so that all areas of the office or building that are being networked are in range of at least one of the WAPs. Then just make sure that all the computers and access points use the same SSID.
One of the current limitations of roaming is that each access point in an ESS must be on the same TCP/IP subnet. That way, a computer that roams from one access point to another within the ESS retains the same IP address. If the access points had a different subnet, a roaming computer would have to change IP addresses when it moved from one access point to another.
Protocols make it possible for the various components of a network to communicate with each other, and standards make it possible for different manufacturers’ network components to work together.
The OSI model is not a networking standard in the same sense that Ethernet and TCP/IP are networking standards. Rather, the OSI model is a framework into which the various networking standards can fit.
Logical addresses are created and used by Network layer protocols, such as IP or IPX. The Network layer protocol translates logical addresses to MAC addresses.
most protocols divide the logical address into two parts: Network address: Identifies which network the device resides on Device address: Identifies the device on that network
TCP is a connection-oriented Transport layer protocol. UDP is a connectionless Transport layer protocol.
The Session layer establishes conversations — sessions — between networked devices. A session is an exchange of connection-oriented transmissions between two network devices. Each transmission is handled by the Transport layer protocol. The session itself is managed by the Session layer protocol.
Application layer represents the programming interfaces that application programs use to request network services.
The Application layer of the TCP/IP model corresponds to the upper three layers of the OSI model — the Session, Presentation, and Application layers. Many protocols can be used at this level. A few of the most popular are HTTP, FTP, Telnet, SMTP, DNS, and SNMP.
allows a single network device to exchange data with another single network device. TCP isn’t used to broadcast messages to multiple network recipients. Instead, UDP is used for that purpose.
But then, some years ago, network administrators realized that they could save the trouble of combining TCP/IP with IPX/SPX and NetBEUI by eliminating IPX/SPX and NetBEUI from the equation altogether. As a result, TCP/IP is not just the protocol of the Internet now, but it’s also the protocol on which most LANs are based.
The Internet traces its beginnings back to a small network called ARPANET, built by the Department of Defense in 1969 to link defense installations. ARPANET soon expanded to include not only defense installations but universities as well. In the 1970s, ARPANET was split into two networks: one for military use (renamed MILNET) and the original ARPANET (for nonmilitary use). The two networks were connected by a networking link called IP — the Internet protocol — so called because it allowed communication between two networks.
Class A addresses are designed for very large networks. In a Class A address, the first octet of the address is the network ID, and the remaining three octets are the host ID.
however, because the first two bits of the first octet are required to be 10, in order to indicate that the address is a Class B address. As a result, a total of 16,384 Class B networks can exist.
The problem with Class C networks is that they’re too small. Although few organizations need the tens of thousands of host addresses provided by a Class B address, many organizations need more than a few hundred. The large discrepancy between Class B networks and Class C networks is what led to the development of subnetting, which I describe in the next section.
Subnetting is a technique that lets network administrators use the 32 bits available in an IP address more efficiently by creating networks that aren’t limited to the scales provided by Class A, B, and C IP addresses. With subnetting, you can create networks with more realistic host limits.
Subnetting provides a more flexible way to designate which portion of an IP address represents the network ID and which portion represents the host ID.
Subnetting lets you select an arbitrary number of bits to use for the network ID.
Those IP address bits that represent the network ID are represented by a 1 in the mask, and those bits that represent the host ID appear as a 0 in the mask. As a result, a subnet mask always has a consecutive string of ones on the left, followed by a string of zeros.
Many firewalls use a technique called network address translation (NAT) to hide the actual IP address of a host from the outside world.
DHCP allows individual computers on a TCP/IP network to obtain their configuration information — in particular, their IP address — from a server.
A better way to assign a fixed IP address to a particular host is to create a DHCP reservation. A reservation simply indicates that whenever a particular host requests an IP address from the DHCP server, the server should provide it the address that you specify in the reservation.
Renewing and releasing leases Normally, a DHCP client attempts to renew its lease when the lease is halfway to the point of being expired. For example, if a client obtains an eight-day lease, it attempts to renew the lease after four days. However, you can renew a lease sooner by issuing the ipconfig /renew command at a command prompt.
You can also release a DHCP lease by issuing the ipconfig /release command at a command prompt. When you release a lease, the client computer no longer has a valid IP address.
If you want to review the complete official specifications for DNS, look up RFC 1034 and 1035 at www.ietf.org/rfc/rfc1034.txt and www.ietf.org/rfc/rfc1035.txt, respectively.
A DNS client can make two basic types of queries: recursive and iterative.
Using the arp command allows you to display and modify the Address Resolution Protocol (ARP) cache, which is a simple mapping of IP addresses to MAC addresses.
Using the ipconfig command displays information about a computer’s TCP/IP configuration. It can also be used to update DHCP and Domain Name Server (DNS) settings.
Renewing an IP lease If you’re having an IP configuration problem, you can often solve it by renewing the computer’s IP lease. To do that, use a /renew switch, like this:
After you release the DHCP lease, you can use an ipconfig /renew command to obtain a new DHCP lease for the computer.
Using the netstat command displays a variety of statistics about a computer’s active TCP/IP connections. It’s a useful tool to use when you’re having trouble with TCP/IP applications, such as File Transfer Protocol (FTP), HyperText Transport Protocol (HTTP), and so on.
Displaying interface statistics If you use an -e switch, netstat displays various protocol statistics, like this:
The nslookup command is a powerful tool for diagnosing DNS problems.
The simplest use of nslookup is to look up the IP address for a given DNS name. For example, how did I know that 66.135.192.87 was the IP address for www.ebay.com? I used nslookup to find out:
Taking a ride through DNS-Land Ever find yourself wondering how DNS really works? I mean, how is it that you can type a DNS name like www.disneyland.com into a web browser and you’re almost instantly transported to the Magic Kingdom? Is it really magic? Nope. It isn’t magic;
Using the pathping Command pathping is an interesting command that’s unique to Windows. It’s sort of a cross between the ping command and the tracert command, combining the features of both into one tool. When you run pathping, it first traces the route to the destination address much the way tracert does. Then, it launches into a 25-second test of each router along the way, gathering statistics on the rate of data loss to each hop. If the route has a lot of hops, this can take a long time. However, it can help you to spot potentially unreliable hops. If you’re having intermittent trouble
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Using the ping Command ping is probably the most basic TCP/IP command line tool. Its main purpose is to determine whether you can reach another computer from your computer. It uses Internet Control Message Protocol (ICMP) to send mandatory ECHO_REQUEST datagrams to the specified host computer. When the reply is received back from the host, the ping command displays how long it took to receive the response.
You can also ping by using a DNS name, as in this example: C:\>ping www.lowewriter.com
Using the route Command Using the route command displays or modifies the computer’s routing table.
Still, if you’re having trouble accessing other computers or other networks, you can use the route command to make sure that a bad entry in the computer’s routing table isn’t the culprit.
Using the tracert Command The tracert command (traceroute in Unix/Linux implementations) is one of the key diagnostic tools for TCP/IP. It displays a list of all the routers that a packet must go through to get from the computer where tracert is run to any other computer on the Internet. Each one of these routers is called a hop, presumably because the original designers of the IP protocol played a lot of hopscotch when they were young. If you can’t connect to another computer, you can use tracert to find out exactly where the problem is occurring.
Understanding how tracert works
