Mr. Mark Welton

 A computer network, or simply a network, is a collection of computers and other hardware interconnected by communication channels that allow sharing of resources and information. Where at least one process in one device is able to send/receive data to/from at least one process residing in a remote device, then the two devices are said to be in a network. A network is a group of devices connected to each other. Networks may be classified into a wide variety of characteristics, such as the medium used to transport the data, communications protocol used, scale, topology, benefit, and organizational scope. - Wikipedia

 A computer network can be defined as two or more computers connected by some means through which they are capable of sharing information  So is sneaker net a network?

 A connection involves some sort of addressing or identification of the nodes on the network, allowing them to speak the same protocol or language.

 Types of networks:  LAN - is a network that is confined to a limited space, such as a building or floor.  WAN - A WAN is a network that is used to connect LANs by way of a third-party provider.  CAN - is a network that connects LANs and/or buildings in a discrete area owned  or controlled by a single entity.  MAN - is a network that connects LANs and/or buildings in an area that is often  larger than a campus.

 We also describe networks by the technology they use ◦ Ethernet ◦ Token Ring ◦ FDDI (Fiber Distributed Data Interface)  Or the Protocols: ◦ ATM ◦ Frame Relay ◦ MPLS ◦ T1 ◦ DS3  Or the layer: ◦ Bridged ◦ Routed ◦ Point-to-point

 Early Ethernet, 10Base-5 used a thick cable named thick-net  10Base-2 Ethernet replaced 10Base-5 and used a smaller coaxial cable named thin-net  Both used either an N or BNC connector to couple cables together and required terminators to be installed at the end of a cable run  These cable runs formed the physical backbone of Ethernet networks

 Ethernet running over UTP cables terminated with RJ45 connectors made use of hubs  Hubs became the new backbone of many installations  As hubs became less expensive, extra hubs were often used a repeaters in more complex networks  Hubs are layer one devices (physical/electrical devices) and do not alter frames or make decisions on traffic it sees

 A hub is simply a means of connecting Ethernet cables together so that their signals can be repeated to every other connected cable on the hub  Hubs may also be called repeaters for this reason, but it is important to understand that while a hub is a repeater, a repeater is not necessarily a hub.

 If another device is already using the wire, the sending device will wait a bit and then try to transmit again  When two stations transmit at the same time, a collision occurs  Each station records the collision, backs off again, and then retransmits  A collision domain is an area  of an Ethernet network where collisions can occur

 A broadcast domain is the area of an Ethernet network where a broadcast will be propagated  Broadcasts stay within a Layer-3 network (unless forwarded), which is usually bordered by a Layer-3 device such as a router  A broadcast storm results in the attempt to build redundancy into the network  The only way to resolve a broadcast storm is to break the loop

 The rule of Ethernet design states that between any two nodes on an Ethernet network, there can be only five segments, connected via four repeaters, and only three of the segments can be populated. This rule, which seems odd in the context of today’s networks, was the source of much pain for those who didn’t understand it.

 Switches differ from hubs by playing an active role in the processing of frames on the network  Switch - The general term used for anything that can switch  Ethernet Switch - Any device that forwards frames based on their Layer-2 MAC addresses using Ethernet  A switch creates a collision domain on each port, while a hub’s collision domain exists on all ports

 Layer-3 Switch - This is a switch with routing capabilities  Multilayer Switch - Similar to a Layer-3 switch, but may also allow for control based on higher layers in packets. Multilayer switches allow for control based on TCP, UDP, and even details contained within the data payload of a packet  Switching - is the act of forwarding frames based on their destination MAC addresses

 Every frame contains the source and destination MAC address  switch inspects the frame to determine the source MAC address and adds the address into its address table  The switch then determines the frame’s destination MAC address and checks the table  If a match exists, the frame is forwarded from the source port to the destination port. If no match is found, the frame is sent to all ports

 Routing - refers to determining the path for something  For IP, packets and frames are forwarded by switches and hubs  To reach a destination, not on the local network, the packet must be forwarded to a gateway  The gateway (router) is responsible for determining how to get the packet to where it needs to go

 Routers communicate with each other by routing protocols  These protocols let the routers learn information about other networks which are stored in the routing table of the router  With the introduction of layer three switches – routers tend to remain WAN-focused, while switches remain LAN-focused

 Routing is a fundamental process common to almost every network  Each routing protocol has its own table of information (learned through its own protocol – in addition to the routing table of the router)  Metrics - the values or weight used by a routing protocol to determine its best path to a destination  Administrative distance is the values assigned to each routing protocol by the router in order to prioritize learned routes

 If the same route is learned from two sources within a single routing protocol, the one with the best metric will win  If the same route be learned from two routing protocols within a single router, the protocol with the lowest administrative distance will be preferred

 When a packet arrives at a router, the router determines whether the packet needs to be forwarded to another network  If it does, the router checks the routing table to determine if it contains a route to the destination network  If there is a match, the packet is forwarded out the proper interface to where it belongs  If there is no match found in the routing table, the packet is forwarded to the default gateway, if one exists, or dropped if no default gateway exists

 IP destination networks are often described by a network address and prefix length  The prefix length is the number of bits set to 1 in the subnet mask  Networks are described in the format network-address/prefix-length  i.e /16  referred to as a prefix

 /24  /16  /8  Which has the longer prefix?  A longer prefix means a more specific match to a destination

 Host route  subnet route  summary route  major network route  supernet route  default route

 Host route is a route to a host (node), not a network  Host routes have a subnet mask of or a /32 prefix  The subnet mask is all 1s

 Subnet route is a portion of a major network  The subnet mask is used to determine the network  24 is a subnet of the classful network  Subnets make use of Variable Length Subnet Masks of classful major networks

 Summary route is a single route that references a group of subnets  /16 would be a summary route for the subnet routes of /24 and /24

 is any classful network with its traditional subnet mask  Class A, B, C networks

 Supernet route is a single route that references a group of major network  /15 is a supernet for /16 and /16

 Default route is the gateway (route) of last resort  typically shown as (an all zero) destination  The subnet mask is all zeroes