Designing Models for Addressing and Naming

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Transcript Designing Models for Addressing and Naming

Designing Models for Addressing and
• 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
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
Using a Structured Model for
Network-Layer Addressing
• Addresses are meaningful, hierarchical, and
• 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
Using Meaningful Network
• 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
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
• Use private addresses as an alternative
Distributing Authority for
• Determine who is to implement model
– if inexperienced networks administrators then
keep it simple
• If branch/regional offices inexperienced
then keep authority centralized
Using Dynamic Addressing for
End Systems
• Reduces the configuration tasks required to
connect end systems
• Supports users who change offices
• Built into desktop protocols such as
AppleTalk and Novell NetWare
• Minimizes configuration tasks
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
doesn’t have to get new address each time it
• AppleTalk station communicates with a
router to determine the cable range for its
network segment
Novell NetWare Dynamic
• Station address consists of 4 byte network
number and a 6-byte node ID.
• 6-byte node ID is the same as the station’s
MAC address
• A network manage configures routers and
servers on a NetWare network with the 4
byte network number for a network
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
– Reverse Address Resolution Protocol (RARP) and
– BOOTP is more sophisticated than RARP and returns
additional information including address of default router
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
The Dynamic Host Configuration
Protocol (DHCP) (Cont’d)
• 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
The Dynamic Host Configuration
Protocol (DHCP) (Cont’d)
• 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
The Dynamic Host Configuration
Protocol (DHCP) (Cont’d)
• 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
Using Private Addresses in an IP
• 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
Using Private Addresses in an IP
Environment (Cont’d)
• Network can advertise just one network
number or small block of numbers to the
• Can reserve scarce Internet addresses for
public servers
Caveats with Private Addressing
• Outsourcing network management is
• Difficulty of communicating with partners,
vendors, suppliers, and other outsiders
• Easy to forget to use a structured model
• Assign in a structured, hierarchical fashion
Network Address Translation
• 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
Network Address Translation
(NAT) (Cont’d)
• Some NAT products offer port translation
for mapping several addresses to the same
• 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
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
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
Why Use a Hierarchical Model for
Addressing and Routing (Cont’d)
• 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
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
Classless Inter-Domain Routing
• The internet has a severe scalability
• 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
Classless Routing Versus Classful
• IP address contains a prefix part and a host
– 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
Classless Routing Versus Classful
Routing (Cont’d)
• 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)
Route Summarization
• 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
Route Summarization Tips
• For route summarization to work correctly
– Multiple IP addresses must share the same leftmost 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
Discontiguous Subnets
• Subnets must be next to each other to be
• Classless routing protocol can be used to
route to discontiguous subnets
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 staticallydefined IP address
• Routers use the longest prefix available that
is appropriate for the destination address in
the packet
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
• It is important to avoid inadvertently
overlapping blocks of addresses
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
Distributing Authority for
• 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
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
Assigning Names in an
AppleTalk Environment
• You assign names to shared servers and
• Use meaningful names
• can also assign names to zones. A zone is a
collection of nodes that share information
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
• Generally no need to assign names to end
Assigning Names in a NetBIOS
• Is a session-layer protocol that includes
functions for naming devices which ensures
the unique of names and finding named
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
NetBIOS in a Novell NetWare
Environment (NWLink)
• With NWLink NetBIOS runs on top of
Novell’s Sequenced Packet Exchange and
Internetwork Packet Exchange protocols
• Uses type-20 broadcast packets to send
name registration
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
Assigning Names in an IP
• Accomplished by configuring hosts files,
DNS servers, or Network Information
Service (NIS) servers.
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