 Medium for communication between entities connected to it  “Entities” are referred to as hosts  Is the Internet a network?  In simplest terms, yes  In reality, it is a network of interconnected networks Networks & Protocols Part III2

3 INTERNET

Networks & Protocols Part III4

 Networks are formed by connecting hosts together  Hosts on the same network form a local area network (LAN)  Multiple networks can be connected to form a wide area network (WAN)  Connecting hosts across a LAN is a function of the Link Layer  Connecting hosts across a WAN is a function of the Internet Layer Networks & Protocols Part III5

 All network interfaces have a unique address “burned into” the electronics of the interface card  Used to distinguish one host from another  Cannot be changed  MAC addresses are 48-bits represented in hexadecimal  01:23:45:67:89:AB  FE-DC-BA  MAC address can be viewed using the ipconfig utility  ipconfig /all Networks & Protocols Part III6

 Networks require a means of forwarding traffic between hosts  Hubs provide the simplest means to connect hosts (Think ball of solder)  Operate at the physical layer  All hosts on network connected via cable (or another medium) to a central hub  Traffic from a host is transmitted (via the cable) to the hub  Traffic is forwarded from the hub to all connected hosts Networks & Protocols Part III7

 Media Access Control (MAC) is a function of the Link Layer  Coordinates access to a shared medium  All hosts connected via a hub  Hosts see all data on the medium  Only processes data if it is addressed to its own MAC address  Ignores all other data (doesn’t process it) Networks & Protocols Part III8

 “Smart” hubs  Forwards packets only to the host they’re addressed to  Maintains a table of which machine is connected to which port  Based on MAC address  Traffic coming in on one port is checked against the table  Only forwarded to the port the recipient is connected to  Operate at the Link Layer  Forwarding data based on MAC address requires link layer processing Networks & Protocols Part III9 PortMAC Address 111:22:33:44:55:66 266:77:88:99:aa:bb 3aa:bb:cc:dd:ee:ff

 Large networks are often broken into smaller networks  Communications within a LAN handled at the Link Layer  Outside of the network requires the Internet layer  How do we know if two hosts are on the same network?  Two hosts on the same network have the same initial sequence of bits in their IP addresses  Common sequence defines which network a host is on  How long is the sequence of bits?  Defined by the 32-bit subnet mask Networks & Protocols Part III10

 Sequence of 1’s followed by a sequence of 0’s  Sequence of 1’s defines the network portion of the IP address  Sequence of 0’s defines the host portion of the address  Subnet mask usually specified in dotted quad notation  Ex: , ,  Alternatively specified as /X  X represents the number of contiguous bits set to 1  Ex:  /8 → IP Address , Subnet Mask:  /16 → IP Address , Subnet Mask:  /24 → IP Address , Subnet Mask:  /22 → IP Address , Subnet Mask:  Network address calculated by ANDing the IP address and subnet mask Networks & Protocols Part III11

 Variant of logical AND  Applies AND operation to two numbers bit-by- bit  true and false → 1 and 0  AND truth table Networks & Protocols Part III12 ABAB

 Special hosts connected to multiple networks  “Routes” data packets from one network to another  Operates at the Internet layer  Communicate with other routers using a routing protocol  Routing protocols used to determine  Which networks the router is connected to  The best route to a given network  Routers only concerned with the “next hop”  Router directly connected to it that has a path to the target network  If multiple paths exist, relative weights can be assigned based on factors such as link type, speed, etc. Networks & Protocols Part III14

 Router that provides access to networks outside of the local network  Host first determines if the destination IP address is on its local network  If so, data is transferred directly to that host  Using the host’s MAC address  Traffic destined outside the local network must be passed to the default gateway  Default gateway determines the best path to the destination network  Forwards the data to the next hop on that path Networks & Protocols Part III15

Networks & Protocols Part III16

Networks & Protocols Part III17