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Fundamentals Of IP

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Fundamentals Of IP

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents


Fundamentals of IP

Routing is synonymous with path selection.


Typical Features of Layer 3

  • Defines routing and addressing
  • PSI defines unique L3 protocol, Connectionless Network Services (CLNS)
  • Other protocols include IP, IPX, AppleTalk Datagram Delivery Protocol (DDP)
  • Routing Table contains list of network layer address groupings
  • One entry per group, tells where to route packet next

Network Layer Interaction w/ Data Link Layer: Network layer analyzes packet, sends it to data link layer, data link layer adds appropriate header/trailer (Ethernet/HDLC/etc), Next router receives, strips header/trailer. Repeat process until destination reached.

ARP (Address Resolution Protocol): Used to dynamically learn data-link (MAC) address of an IP host.

IP - > MAC


Network Layer Addressing

  • Network addresses
  • TCP/IP = Network or subnet
  • IPX = Network
  • AppleTalk = Cable range
  • Network ID labels network; Host ID labels specific member within that network
  • Can have duplicate host IDs, as long as different network IDs
Protocol Size (Bits) Grouping Field (Bits) Address Field (Bits)
IP 32 Network (8-30) Host (2-24)
IPX 80 Network (32) Node (48)
AppleTalk 24 Network (16) Node (8)
OSI Variable Many formats Domain-specific


'Routing Protocols ': End goal is to fill the routing table with all known destination groups and w/ the best route to reach each group.

  • All IP addresses in same group must not be separated by router
  • IP addresses separated by router must be in separate group

Classes of Networks

Class 1st Octet Range # of Network Bytes # of Host Bytes # of Addresses per Network
A 1 -126 1 3 224 - 2
B 128 - 191 2 2 216 - 2
C 192 - 223 3 1 28 - 2

Reserved addresses = Network Address and Broadcast Address


Address Formats When Subnetting is Used Image:Addressing-Formats.GIF

Network Layer Utilities

Domain Name System (DNS): Send DNS request, resolves IP address of machine

Address Resolution Protocol (ARP): Resolve MAC Address by sending ARP broadcast with specific IP

ICMP Echo: Used in ping command, computer tested replies w/ a ICMP echo reply.

Reverse ARP (RARP): Broadcast IP address, resolve ONLY the MAC Address

Boot Protocol (BOOTP): Broadcast IP address, resolve MAC Address, subnet mask, default gateway, other server IP addresses, and name of a file that should be downloaded

- > Main goal is to remotely booting an operating system

*Administration difficult for the above 2 because server had to know every MAC address of every computer.


Dynamic Host Control Protocol (DHCP): Dynamically assign IP from range of available IP addresses.


IP Routing Protocols: Includes subnet number, the interface which to forward packets (Serial, Ethernet, etc.), and the Next Hop IP Address.


  • Dynamically learns and fills the routing table
  • Place best route (if more than 1) in table
  • Remove obsolete routes, add new routes.
  • Time between losing a route and adding a new one = Convergence Time
  • Try to prevent routing loops


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