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Thursday, December 31, 2009

Day 28: Network Diagrams and Topology

A physical topology displays actual device and wiring locations to help you efficiently locate and troubleshoot devices - we have bus, ring, mesh, star or extended star topologies. It maps the location of OSI Layer 1 devices and media. So it refers to the physical layout of devices and network media.
A logical map of the network topology groups hosts and devices by how they use the network. A logical topology map displays hostnames, address groups, network access and applications on a network. The logical topology identifies the Layer 1 devices but focuses on the Layer 3 addressing, access and upper-layer applications, regardless of location. It refers to the logical paths in which data accesses the media and transmits packets accross it. Ethernet uses a logical bus topology and either a physical bus or star topology.

The backbone for the Internet are IXPs (Internet Exchange Points) and NAPs (Network Access Points), ISPs use these to connect to each other.

An Ethernet address or media access control (MAC) address provides a unique identity for a host but does not provide information about the host's location, typically burned into the adapter and usually displayed in a hexadeximal format. The first 24 bits are vendor specific (OUI), the other are vendor assigned. The Internet Protocol (IP) address identifies the location of a host in a divided hierarchical network - remember: access, distribution and core layer.

A hub is a multiport device that simply regenerates a received signal to all ports except the port where the signal is received. The bandwith is shared and only one device can communicate at a time. All connected devices are in the same collission domain. Even with a collision will the hub forward the frame with errors out all ports where it'll be discarded by the NIC.

A switch is also a multiport device that reads each frame's MAC address, maintains a MAC table of which hosts are attached to which port, and forwards frames based on the destination MAC address. Bandwith is not shared between the switch creates temporary circuits, so each port is its own collision domain! When a MAC is not yet learned the swith'll flood the frame out all other ports. Frames with errors or with same source and destination will not be forwarded.

Switches have these advantages over bridges:
  • a high-speed backplane that enables multiple simultaneous conversations to occur.
  • data-buffering capabilities that store and forward packets to the correct ports or port.
  • higher port densities versus bridges.
  • lower latency than bridges. Layer 2 switches are implemented in hardware, allowing millions of bits per second to be transmitted at the same time.
A network with switches and hubs form a single broadcast domain. Broadcast messages (to all other hosts on a network) use all F's as the destination MAC address. When only the IP address of the destination is known , the sending host can use address resolution protocol (ARP) - also check my other post concerning gratuitious arp. ARP is a local broadcast sent to all devices on the local segment to find the MAC address of a host.

Remember these things about routers:
  • they connect networks and route packets to their destination networks.
  • routers can look at the MAC address to determine a frame's destination, but also decapsulate the frame to look at the destination IP address located in the header of the IP packet.
  • routers look at the network portion of the destination IP address, re-encapsulate the packet and forward it to its destination.
  • routers maintain a routing table of connected networks, it's referenced to determine which interface connects to thet destination network.
  • routers do not forward frames with a broadcast MAC address, so each port is its own broadcast domain. It's clear now that routers divide broadcast domains.
  • routers drop packets when it has no entry for a destination network (use a default route).
A Lan segment is a network connection made by a single unbroken network cable, they are limited by physical distance. You can extend it by using hubs, repeaters, bridges and switches.

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