CIS460

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CIS460 NETWORK ANALYSIS AND DESIGN

• CHAPTER 6
• Designing Models for Addressing and Naming

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Overview

• Look at the guidelines for assigning addresses
  and names to internetwork components, including
  networks, subnets, routers, servers, and end
  systems
• Importance of using a structured model for
  network layer addressing and naming
• Importance of developing policies and procedures
  for addressing and naming

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Guidelines for Assigning Network-Layer Address

• Should be planned, managed and documented
• Rules for network-layer addressing
• Design a structured model
• Leave room for growth
• Assign blocks of addresses in a hierarchical
  fashion
• Use meaningful numbers
• Delegate authority if possible to regional/branch
• use dynamic addressing
• use private addresses with network address
  translation

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Using a Structured Model for Network-Layer
Addressing

• Addresses are meaningful, hierarchical, and
  planned
• A clearly documented structured model facilitates
  management and troubleshooting
• With no model problems can occur
• Duplicate network and host addresses
• illegal addresses that cannot route on internet
• insufficient addresses
• addresses that cannot be used

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Using Meaningful Network Numbers

• In AppleTalk assign cable range for each network
  segment
• e.g., building number/floor number
• In Novell NetWare each network segment is
  assigned a 4-byte hexadecimal number
• Using building and floor numbers allow
  identifying and narrowing location of problems

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Administering Addresses by a Central Authority

• Corporate IS or enterprise network department
  should develop a global model for network-layer
  addressing
• In an IP environment can request block of numbers
  from an ISP or Internet Assigned Numbers
  Authority
• If from ISP request a large enough block for
  scalability
• Use private addresses as an alternative

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Distributing Authority for Addressing

• Determine who is to implement model
• if inexperienced networks administrators then
  keep it simple
• If branch/regional offices inexperienced then
  keep authority centralized

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Using Dynamic Addressing for End Systems

• Reduces the configuration tasks required to
  connect end systems
• Supports users who change offices frequently
• Built into desktop protocols such as AppleTalk
  and Novell NetWare
• Minimizes configuration tasks

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AppleTalk Dynamic Addressing

• Network layer stations address consists of a
  16-bit network number and an 8-bit node ID
• Once network-layer address is chosen it is saved
  in battery-backed-up RAM so it doesnt have to
  get new address each time it boots
• AppleTalk station communicates with a router to
  determine the cable range for its network segment

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Novell NetWare Dynamic Addressing

• Station address consists of 4 byte network number
  and a 6-byte node ID.
• 6-byte node ID is the same as the stations MAC
  address
• A network manage configures routers and servers
  on a NetWare network with the 4 byte network
  number for a network segments

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IP Dynamic Addressing

• An IP layer address is 4 bytes in length and
  consists of a prefix and host part
• In the past each host was required to be
  configured manually. Now addressing is done
  dynamically through
• Reverse Address Resolution Protocol (RARP) and
  BOOTP
• BOOTP is more sophisticated than RARP and returns
  additional information including address of
  default router

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The Dynamic Host Configuration Protocol (DHCP)

• DHCP is based on BOOTP
• BOOTP hosts interoperate with DHCP
• DHCP adds many enhancements to BOOTP including
  larger vendor specific information field and
  automatic allocation of reusable network-layer
  address
• DHCP uses a client/server model
• Servers allocate network-layer addresses and save
  information about which addresses are used

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The Dynamic Host Configuration Protocol (DHCP)
(Contd)

• DHCP supports three methods for IP address
  allocation
• Automatic allocation - assigns permanent IP
  address to a client
• Dynamic allocation - assigns an IP address to a
  client for a limited period of time
• Manual - network administrator assigns
  permanently and DHCP merely conveys address
  information

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The Dynamic Host Configuration Protocol (DHCP)
(Contd)

• Dynamic is most popular where hosts are not on
  all of the time. Address given for a short
  period time called a lease
• Can reuse address if lease has expired
• Chen client boots it broadcasts a DHCP discover
  message on its local subnet
• each server responds with a DHCP offer message

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The Dynamic Host Configuration Protocol (DHCP)
(Contd)

• Client choose one server response to request
  configuration parameters
• Server selected commits configuration parameters
  to persistent storage and responds with DHCP ACK
  message
• If no response received client times out and
  resends a DHCP discover and request messages

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Using Private Addresses in an IP Environment

• These are addresses assigned by internal networks
  and hosts without any coordination from an ISP or
  the Internet Assigned Numbers Authority
• An advantage is security. Private numbers are
  not advertised on the Internet
• Helps meet goals for adaptability and flexibility

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Using Private Addresses in an IP Environment
(Contd)

• Network can advertise just one network number or
  small block of numbers to the Internet
• Can reserve scarce Internet addresses for public
  servers

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Caveats with Private Addressing

• Outsourcing network management is difficult
• Difficulty of communicating with partners,
  vendors, suppliers, and other outsiders
• Easy to forget to use a structured model
• Assign in a structured, hierarchical fashion

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Network Address Translation (NAT)

• An IP mechanism that is used for converting
  addresses from an inside network to addresses
  that are appropriate for an outside network and
  vice-versa
• NAT administrator configures a pool of outside
  addresses that can be used for translation

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Network Address Translation (NAT) (Contd)

• Some NAT products offer port translation for
  mapping several addresses to the same address
• When using NAT all traffic must go through a NAT
  gateway
• Must also modify IP addresses that occur inside
  the data part of a packet

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Using a Hierarchical Model for Assigning Addresses

• Hierarchical addressing is a model for applying
  structure to addresses so that numbers in the
  left part of an address refer to large blocks of
  networks or nodes, and numbers in the right part
  of an address refer to individual networks or
  nodes

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Why Use a Hierarchical Model for Addressing and
Routing

• Support for easy troubleshooting, upgrades and
  manageability
• Optimized performance
• Faster routing-protocol convergence
• Scalability
• Stability
• Fewer network resources need

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Why Use a Hierarchical Model for Addressing and
Routing (Contd)

• Permits summarization (aggregation) of network
  numbers
• Summarization allows a router to group many
  network numbers when advertising its routing
  table
• Facilitates variable-length subnet masking (VLSM)

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Hierarchical Routing

• Means that knowledge of the network topology and
  configuration is localized
• No single router needs to understand how to get
  to each other network segment
• Addresses must be assigned in hierarchical fashion

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Classless Inter-Domain Routing

• The internet has a severe scalability problem
• Classless inter_domain Routing (CIDR) is a method
  for summarizing routes
• Addresses should be assigned in blocks
• Routers should group routes together to cut down
  on the quantity of routing information shared by
  Internet routers

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Classless Routing Versus Classful Routing

• IP address contains a prefix part and a host part
• Prefix identifies a block of host numbers and is
  used for routing that block
• Traditional (classful routing) does not transmit
  any information about the prefix length
• Traditional IP hosts and routers had a limited
  capability to understand prefix lengths and
  subnets

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Classless Routing Versus Classful Routing (Contd)

• Classless routing protocols transmit a prefix
  length with an IP address
• Classless routing protocols include Routing
  Information Protocol (RIP) V2, Enhanced Interior
  Gateway Routing Protocol (Enhanced IGRP), Open
  Shortest Path First (OSPF), Border Gateway
  Routing Protocol (BGP) Intermediate System-to
  Intermediate System (IS-IS)

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Route Summarization (Aggregation)

• When advertising routes into another major
  network classful routing protocols automatically
  summarize subnets
• Only advertise route to a Class A, B, or C
  network instead of routes to subnets
• Discontiguous subnets are not supported
• Classless routing protocols advertise a route and
  a prefix length

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Route Summarization Tips

• For route summarization to work correctly
• Multiple IP addresses must share the same
  left-most bits
• Routers must base their routing decisions on a
  32-bit IP address and prefix length that can be
  up to 32 bits
• routing protocols must carry the prefix length
  with 32-bit addresses

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Discontiguous Subnets

• Subnets must be next to each other to be
  supported
• Classless routing protocol can be used to route
  to discontiguous subnets

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Mobile Hosts

• Classless routing and discontiguous subnets
  support mobile hosts
• A mobile host is a host that moves from one
  network to another and has a statically-defined
  IP address
• Routers use the longest prefix available that is
  appropriate for the destination address in the
  packet

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Variable-Length Subnet Masking

• Using classless routing means that you can have
  different sizes of subnets within a single
  network
• Variable length subnet masking (VLSM) relies on
  providing prefix length information explicitly
  with each use of an address
• It is important to avoid inadvertently
  overlapping blocks of addresses

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Designing a Model for Naming

• Short meaningful names enhance user productivity
  and simplify network management
• A good naming model strengthens the performance
  and availability of a network
• It should let a user transparently access a
  service by name rather than address
• The system should map the name to the address

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Distributing Authority for Naming

• No department should be burdened with assigning
  and maintaining all names
• If device has local name server instead of
  depending on a centralized server many names can
  be resolved to addresses locally without causing
  traffic on the internetwork

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Guidelines for Assigning Names

• Should be short, meaningful, unambiguous and
  distinct
• Users should recognize which name goes with which
  device
• Can use three letter prefixes
• Some networks use geographical names
• Avoid names with unusual characters
• Avoid case sensitivity

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Assigning Names in an AppleTalk Environment

• You assign names to shared servers and printers
• Use meaningful names
• can also assign names to zones. A zone is a
  collection of nodes that share information

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Assigning Names in a Novell NetWare Environment

• Assign names to resources such as volumes on a
  file server, shared printers, print queues,
  printer servers and possibly other servers
• Generally no need to assign names to end systems

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Assigning Names in a NetBIOS Environment

• Is a session-layer protocol that includes
  functions for naming devices which ensures the
  unique of names and finding named services

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NetBIOS in a Bridged or Switched Environment
(NetBEUI)

• NetBIOS was originally implemented as session
  layer software that runs on top of the driver for
  a NIC
• Makes extensive use of broadcast packets for
  naming functions
• When started it broadcasts check-name queries to
  make sure its name is unique
• Can have as many as 32 names

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NetBIOS in a Novell NetWare Environment (NWLink)

• With NWLink NetBIOS runs on top of Novells
  Sequenced Packet Exchange and Internetwork Packet
  Exchange protocols
• Uses type-20 broadcast packets to send name
  registration

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NetBIOS in a TCP/IP Environment (NetBT)

• In this protocol you have 4 options for name
  registration and lookup
• Broadcasts - used to announce services
• Lmhosts files - a lmhosts file is place on each
  station. Not dynamic
• Windows Internet Name Service (WINS) - use WINS
  server to resolve a name
• Domain Name System (DNS) - standard internet
  service which uses a generic IP environment

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Assigning Names in an IP Environment

• Accomplished by configuring hosts files, DNS
  servers, or Network Information Service (NIS)
  servers.

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The Domain Name System

• Developed in early 1980s to manage a hosts file
  containing the names and addresses of all the
  systems on the Internet
• com, edu, gov, etc
• Can also include geographical level domains