The document discusses network assessment and administration. It describes how network messages are broadcast, defining broadcast messages as messages addressed to every host on a network. It also discusses system administration, describing it as where humans interface with computers to solve problems or provide services. The document outlines various components of a computer network, such as humans, hosts, routers, switches, and cables. It provides steps for network analysis, including reviewing physical network topology, DNS data, operating systems, configurations, devices, subnets, routers, IP addresses, and services used on the network.

1. Network and System
Administration
Network Assessment
Lesson 01
COIS 435

2. Network Assessment!
• Every network message on an Ethernet
segment reaches every host on that segment,
but usually only the machine with the
destination IP address listens. A broadcast
message is a message, which is addressed
to every host on a network. The usual
convention for broadcast messages is that all
the host address bits one wishes to reach are
set to ones, e.g. 128.39.89.255."
© 2007 Angel G. Diaz. All Rights Reserved. 2

3. System Administration
• System administration is where humans meet
the computer. It is right to talk about a
Human-Computer System."
• A Human-Computer System is an organized
collaboration between humans and
computers to solve a problem or provide a
service. Although computers are
deterministic, humans are non-deterministic,
so human-computer systems are non-
deterministic."
© 2007 Angel G. Diaz. All Rights Reserved. 3

4. Components of a Computer Network
• Humans: who use and run the fixed infrastructure, and cause
most problems."
• Hosts: computer devices that run software. These might be in a
fixed location, or in mobile devices."
• Routers: computing devices that direct traffic around the
Internet. Routers talk at the IP address level, or `layer 3',
simplistically speaking."
• Switches: fixed devices that direct traffic around local area
networks. Switches talk at the level of Ethernet or `layer 2'
protocols, in common parlance."
• Cables: There are many types of cable that inter-connect
devices: fiber optic cables, twisted pair cables, null-modem
cables, etc."
© 2007 Angel G. Diaz. All Rights Reserved. 4

5. Network Analysis
• If you become a system administrator somewhere, you will have to get to grips
with the local network setup there. There are two main things:"
• Physical devices: cables, routers, repeaters...subnets"
• Services: what is the function of each host in the network?"
• There are two kinds of machine in a network: clients and servers.
Servers offer services and clients make use of the services. We need to
find out which hosts do what. This is partly operating system
dependent."
• Windows, NT or Macintosh: Each user has a workstation. Only the
person sitting at the machine can use it. Centralized servers make
shared resources like disks and printers available over the network."
• Unix, IBM OS or Cray OS: Several users can use each host by remote
login. Any machine can be a server or a client or a workstation. No
special software is needed.
© 2007 Angel G. Diaz. All Rights Reserved. 5

6. Network Analysis
How to proceed!
To get an overview of the network you will need to put together information from several
sources like a jigsaw. These are some of the sources you can use:"
"
• Physical Network!
– You will need a sketch of the topology of the network cables. This has to come from the
people who wired up the network, since only they know what they did."
• DNS Data!
– Start with the DNS database, either in their source file form or use nslookup. Here you
should find all of the IP addresses, which belong to the domain. "
– From the DNS you will be able to find out what the local domain's name server (NS) is
and also where E-mail is sent (MX=Mail Exchanger). You can also find out the mail
address of a person responsible for the network and other information. If a network is
shielded by a firewall, this information is not available from outside the local network.
© 2007 Angel G. Diaz. All Rights Reserved. 6

7. Network Analysis
• Operating System (Version)!
– It is important to know what kind of operating systems are running on your network.
If something goes wrong you need to know who to contact in order to get the
machine repaired. Sometimes dangerous bugs are discovered and you need to
know whether you are vulnerable to these bugs. The command uname –a or
uname –help is used on modern Unix systems. On older systems you will have to
use something like arch. NT has to be identified visually since it is not possible to
log onto the machine remotely. In some cases you will be able to use SNMP tools
like scotty to discover hardware information about your network, including printers,
routers and other network devices."
• Net Configuration!
– Get the netmask, broadcast address and default router. The command ifconfig –a is
used for Unix. The command ipconfig on NT. This command is also used to set the
hosts IP address when it is booted.
© 2007 Angel G. Diaz. All Rights Reserved. 7

8. Network Analysis
• Device Overview!
– We can gain some insight into a machine's hardware using the dmesg command.
This shows the system boot messages. How many CPUs does the machine have,
have many disks and network interfaces. (See also netstat -i on some unixes) This
information lies in a binary file (unusual for Unix), which has the disadvantage that
the file can become corrupt if there is a kernel panic. When this happens running
dmesg usually wrecks your terminal."
• Subnets!
– I many cases a network will consist of several small network segments called
subnets. It is important to map out these subnets because hosts which lie on
different subnets will not necessarily be able to see one another for certain network
services. The thing, which is important to know about subnets, is the netmask and
default route because these two things decide which hosts will see which
messages.
© 2007 Angel G. Diaz. All Rights Reserved. 8

9. Network Analysis
• Routers!
– Routers join together networks and subnets. Each router has an address on every
network it is connected to. Try looking up nslookup mail.yahoo.com."
– Broadcast messages so not usually go through routers to other subnets, unless
they are so-called directed broadcasts to different physical parts of a common
network. (This means that certain services like BOOTP and DHCP usually do not
work across subnet boundaries.)"
• IP Address!
– A host's IP address is set when it boots by calling the ifconfig command. There are
two ways in which an IP number can be assigned. We can decide once and for all
which address the host should have (out of the addresses we have at our
disposition). The address is set during the installation of the host and it is stored on
disk forever more. The other way is to ask a server to provide the host with an
unused IP address. This is BOOTP/DHCP. This method is used by hosts, which do
not have their own disks for instance. When such a host boots it sends a broadcast
message to the whole subnet asking for some server to tell it what it's address
should be. An active server will reply with an IP address and then mark that
address as currently in use.
© 2007 Angel G. Diaz. All Rights Reserved. 9

10. Network Analysis
• Services!
– After mapping out the hardware configuration of the network
there is still the issue of software systems. This includes all
of the network services a host is responsible for. There is no
easy or obvious way to find out this information, but on a
well-designed network there are several things, which can
help us. A naming convention is common. Use the DNS to
lookup the following names:"
– empresas.suagm.edu"
– mailhost.empresas.suagm.edu"
– dns.empresas.suagm.edu"
– ftp.empresas.suagm.edu"
© 2007 Angel G. Diaz. All Rights Reserved. 10

11. Network Case
© 2007 Angel G. Diaz. All Rights Reserved. 11

12. © 2007 Angel G. Diaz. All Rights Reserved. 12

13. End of Lesson
© 2007 Angel G. Diaz. All Rights Reserved. 13