This document discusses servers and services in a computer network. It describes the similarities between client and server operating systems like Windows and Linux. Servers can take on different roles to provide services like file storage, printing, email, and more. The document explains DHCP and how it automatically assigns IP addresses to devices on the network. It also discusses the advantages of using server hardware designed for high availability, performance, and reliability over desktop machines.

1. Lecture 2:
Servers and Services
Network Design & Administration

2. Servers vs. Clients
• Microsoft XP, Vista, Win 7 and Server 2008/R2 are similar
architecturally
• All based on the NT kernel architecture
• Concept of Kernel and HAL common to most resent OS’s from
Microsoft:
Network Design & Administration
• Kernel
• Acts a bridge between user applications and hardware
• Manages computer resources
• E.g. process, memory & device management
• HAL (Hardware Abstraction Layer)
• Abstract layer between hardware and operating system
• Hardware specific code
• Linux uses similar concepts but slightly different names (kernel
and board support package) 2

3. XP/Server 2003 Architecture
Diagram[1]
Network Design & Administration
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4. Vista/7 Architecture Diagram[1]
Network Design & Administration
4

5. Gnu/Linux [2]
Network Design & Administration
Board Support
Package (BSP)
Plugins to support
different file systems
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6. Different Server Roles
• File server
• Centralised storage
• Better control/backup/recovery
• Print server
Network Design & Administration
• Spool jobs
• Track usage
• Application server
• Web services e.g. Internet Information Services (IIS)
• Databases e.g. SQL Server
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• Accounting packages
• etc…

7. Different Server Roles
• Mail server
• Terminal Services server
• Remote Access / VPN server
Network Design & Administration
• Hyper-V server
• Domain controller
• Vital within an Active Directory Domain!
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8. Different Server Roles
• DNS Server
• Resolves names
• DHCP server
Network Design & Administration
• Allocated IP address leases
• Streaming media server
• WINS server
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9. Dynamic Host Configuration
Protocol
• How does your network know what the IP
addresses are?
• You could set up static IP address
• Question: Why would this not be a good way to assign
Network Design & Administration
IP addresses?
• Better if something (apart from you) could
handle this and allocate IP addresses on request
• This is what DHCP does (and has done since
1993)
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10. DHCP
The DHCP process works as follows:
1. Client machine boots and broadcasts a DHCP Discovery
query, searching for the server.
2. DHCP Server reserves an IP address for the client and sends
Network Design & Administration
this as a DHCP Offer.
3. Client takes up offer and must tell any other DHCP Servers of
this offer via a broadcast DHCP Request including the IP
address of the server making the offer.
4. Other DHCP Servers cancel their offers to the same client
and return the IP address to the pool, while the accepted
server completes the process by sending a DHCP Ack
message to the client, with addition info e.g. lease expiry 10
time.

11. DHCP
• Note that the Discovery and
Request must be broadcast in
order to inform any other DHCP
Servers of the state of the
transaction.
Network Design & Administration
• The client does not know the
addresses of any DHCP Server, so
it has to broadcast on the
physical subnet to find them.
• (Network administrators may
allow DHCP traffic to be
forwarded by a router onto
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another subnet)

12. DHCP
• In large networks it is usual to have more than one DHCP
Server.
• Question: Why do you think more than one DHCP server should
be present within a network?
• Therefore, need to decide how address ranges are to be
Network Design & Administration
configured
• Overlapping address ranges allows all address available even if
one server goes down.
• Question: What problem does that introduce?
• Could get a conflict
• Solutions:
• DHCP Server Conflict Resolution
• Use non-overlapping ranges 12
• Client starts Address Resolution Probe (ARP) to validate address

13. DHCP
• Address allocation has a few variants[2] –
• Dynamic: the address is allocated for a lease period from
a pool and then re-used after the lease has expired.
• Question:
Network Design & Administration
• What lease would be appropriate
• In the office?
• On the Dell production line where they are checking PC’s
before shipping?
• Automatic: the address is assigned permanently to a
client and the client is preferentially given the same
address next time it asks.
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• Static: a list of MAC/IP address pairs is used to assign to
the client.

14. DHCP
• Microsoft seem to have slightly different definitions of
these terms[4]:
• Static – certain machines (e.g. DHCP, DNS, WINS
Servers, Print Server, Firewall, Router) have defined
addresses which are also excluded from a dynamic
Network Design & Administration
range (also called permanent lease)
• Client Reservation (as above, but for ordinary clients)
• Automatic (Automatic Private IP Addressing, APIPA) –
if DHCP Server unavailable, client can configure itself
in the 169.254.0.1 to 169.254.255.254 range and talk
to other clients in the same range. Client polls
regularly (but not frequently) for a DHCP Server to
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return, to get back to normal.

15. Why use a Server?
• Why not use a desktop machine running Windows 7?
• Hardware reasons:
Engineered
Network Design & Administration
to higher Extensible
standard
Server
Hardware Processors
High
Availability
High 15
performance
I/O

16. Next Time & References
• More on Servers and services
[1] “Modern Operating Systems”, Andrew Tanenbaum, 2008
[2] http://www.ibm.com/developerworks/linux/library/l-linux-
Network Design & Administration
kernel/
[3] Wikipedia, man pages for dhcpd
[4] "Windows Server 2008 - TCP/IP Fundamentals for Microsoft
Windows", eBook available at:
http://www.microsoft.com/download/en/details.aspx?displayla
ng=en&id=8781
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