Network TroubleShooting: Glossary & Appendix

For those people who are unfamiliar with the UCSC network environment, some of the terms in the network troubleshooting guide may not be clear. Listed here are the key terms that may be unclear and a brief description of some of the tools and processes that may help you understand what is happening in certain steps.

AppleTalk AppleTalk is a networking protocol primarily used for communications between Macintosh computers and Apple printers. Appletalk is not routed on the campus network after January 2002. Appletalk can continue to be used on local subnets thereafter, but Appletalk zones will not be visible in the Chooser. (Clients generally access the AppleTalk services through the Chooser.)

Big Brother Big Brother is a service monitoring tool provided by ITS Distributed Computing Group and Network Operations: status.ucsc.edu/bb. The status of services on ITS web, mail, unix and other servers can be quickly determined from the display.

DHCP Dynamic Host Configuration Protocol, or DHCP, allows machines to obtain a legal IP address without the user of the machine making a specific request to ITS staff. Under DHCP, a machine's IP address may change each time the machine restarts, so servers must still be registered with Network Operations to ensure a fixed IP address.

DNS Domain Name Services (DNS) provides a mechanism that allows clients to remember logical machine names rather than IP addresses . DNS provides the mapping between a machine name (e.g., www.ucsc.edu) and its IP address ( e.g., 128.114.143.26). The nslookup command on UNIX can be used to determine a machine's IP address given its name and vice-versa.

Ethernet Ethernet is a hardware and software specification used for much of the campus network. All personal computers and servers on the campus network should have 10BaseT or 100BaseT connectivity built-in.

Internet Weather Campus connectivity to the Internet is from several sources, and understanding the Internet Weather report completely is beyond the scope of this documentation. However, if several clients have reported that they can connect to campus, but not to off-campus web pages, the Internet Weather Report can confirm that our connectivity is impaired. For example, if there is packet loss from more than half of the ISPs on the list, then chances are the campus has lost Internet Connectivity.

IP or TCP/IP Transmision Control Protocol/Internet Protocol is the network protocol used to network computers on the Internet. IP devices each have a unique IP address . On the UCSC network, the IP address identifies the IP network number, subnet number and node number of the desired machine.

IP Address An IP address is a series of four numbers separated by dots ('.'), each of which is between 0 and 255. An IP address must be unique to a machine or the network will not be able to properly delivery network information to that machine. An IP address is made up of network number, a subnet number and a node number.

IP Network Number UCSC's IP administrative network number is 128.114. The ResNet IP network number is 169.233.

Subnet Number The subnet number is defined by ITS/Network Operations. Every machine on a subnet has an IP address that begins with the campus network number followed by the subnet's number. For example, my machine is on the '2' subnet, meaning that its IP address must begin with 128.114.2. Node Number or Device Number The node number identifies the machine of interest. So long as it is unique on the subnet, almost any IP address is sufficient. To ensure that every node number is unique, Network Operations offers DHCP services. If your machine cannot take advantage of DHCP services, you must request a unique IP address from Network Operations.

Nocol Nocol is a ITS/Network Operations service which reports routers, switches, and other network devices which do not respond to queries. Usually means that the target machine is down or unreachable.

nslookup nslookup is a UNIX command that queries a DNS server for machine name and address information. To use nslookup on the unix machines, type "nslookup" followed by an IP address or a machine name. nslookup will return the name, all known IP addresses and all known aliases (which are just alternate names) for the identified machine. As an example: % nslookup www2.ucsc.edu Server: ns1.ucsc.edu Address: 128.114.142.6 Name: bombalurina.ucsc.edu Address: 128.114.129.41 Aliases: www2.ucsc.edu The first line is the command as typed at the UNIX prompt and the next two lines indicate that ns1.ucsc.edu was the machine that nslookup asked for information. The last three tell us that www2.ucsc.edu is actually just an alias for the machine bombalurina.ucsc.edu and the machine's IP address is 128.114.129.41.

ping ping is a simple program that attempts to make a very basic kind of connection to a target machine. If the target machine responds ping alerts you that it is "alive", otherwise ping returns a message that the machine is unreachable. Note that the ability to ping a machine does not mean that the machine is fully functional, but it does indicate that the network path to that machine (including all routers between you and the machine) are up and functioning.
On the Athena machines, ping is located in /usr/etc/ . Ping is also available for Mac/Win machines. Here is a sample invocation of ping: % /usr/etc/ping cats.ucsc.edu cats.ucsc.edu is alive

Protocol

A network protocol describes the "language" that a computer speaks on a network -- the actual structure of the data contained in the network packets. Example protocols in this sense include TCP/IP , Appletalk, and IPX.

Router or Gateway

A router is the device that connects subnets together. Any network packets that are destined for a machine on a different subnet are given to the subnet's router. The router examines the address of the target machine and (if it is a protocol that the router recognizes) "routes" the packet to the subnet where the target machine is located. In going from one subnet to another, the packet may need to go through several routers. The "target router" -- the router for the subnet where the target machine is located, receives the packet and forwards it to the actual target machine.
A client's machine must be configured to know the IP address of the router for its subnet in order to communicate with machines on other subnets. Mis-identified gateways are a common problem for manually configured IP settings.

For purposes of testing the network from a remote station, it is useful to know that at UCSC router addresses generally are a name associated with the physical location of the subnet with "-g" appended. Sample routers on campus are "emscat-g" and "comm-g" (comm stands for "Communications). ping ing or telneting to a router is often a useful step in tracking down problems on a network.

While sometimes it is useful to "ping" a router to locate trouble, ITS/Network Operations provides a service called " Nocol " which displays routers and other network devices which are unreachable (not answering pings) from the Communications building. If you suspect that a network problem may be related to a particular router or switch (e.g. an entire building seems to be without an IP network connection), then check Nocol to see if it is down.

Subnet

A subnet is a section of the campus' TCP/IP network. Each subnet has a unique subnet number . Each subnet is then connected to a router which effectively connect it to all other subnets on campus.

Twisted Pair (10BaseT)

Twisted pair, or 10BaseT, cabling looks similar to a standard RJ-11 modular phone jack. The primary difference is that a 10BaseT cable has an RJ-45 connector which is the same basic shape but slightly larger than a phone cable. 10BaseT transceivers (the box or internal card that connects to the ethernet network) have a "Link Light" -- a green LED that shines if the transceiver both has power and has made positive contact with another network device (hub or switch). A shining link light means that the connection to the network is active. This does not rule out the possibility that there might be a physical problem with the client's specific network connection, but it does mean that such a problem is somewhat less likely to be the cause of a network failure. 10BaseT transmits data at 10 Mbits/sec. 100BaseT, aka "Fast Ethernet" has a bandwidth of 100Mb/s.