In this lesson, we define a network more formally, and discuss how packets move within networks and how they move to other networks.
|⇐||Move your mouse over the cloud to see what our view of networks will be like by the end of this lesson.|
|Our current view of the Internet and computer networks|
ipconfigin a Windows command prompt or
ifconfigin a UNIX terminal.
falseare replaced with
0, respectively. The adjacent truth table defines the result of A•B.
0(one of two possibilities), the total number of unique numbers is given by 28. Two IP addresses in every network are reserved, therefore, the total number of available IP addresses of the example network is 28 - 2 (see note). The more bits in the subnet mask, the fewer host IPs can fit on the network. The fewer bits in the subnet mask, the more host IPs can fit on the network.
1's. A host requesting networking configuration service using DHCP uses the broadcast IP Address, for example.
arprequires an Administrator shell.
arp -ain a Windows command shell. You should see a few tables that map IP addresses to MAC addresses. There is one table for each of your network devices. There are two types of entries, static and dynamic. Static entries are pre-programmed for certain purposes, and are not of interest in this course. Dynamic entries, on the other hand, show the MAC addresses of your host's neighbors; your host "learns" these addresses as it sends packets back and forth, and stores them in this table, which is called an "ARP Table". In this table, you should have an entry for your gateway router and entries for each other host on your network. There should also be a static entry for your network's "broadcast" address, the highest IP address in the range of available IP addresses on your network. The MAC address for this entry should be FF:FF:FF:FF:FF:FF. Packets sent to the broadcast IP address actually are transmitted to all your neighbors, i.e., all the hosts on your network (as defined above).
The following listings show the outputs of
arp from the same host (
Notice that the host (
mich300csd02w) on the same local network (Link Layer) is a single hop away, or directly connected from the perspective of the Network Layer, and that the host (
mich392csd02w) on a separate network is multiple hops away (Network Layer).
Additionally, notice that the host (
mich300csd02w) on the same local network (Link Layer) is in the host's (
mich300csd01w) own ARP table, and how the host (
mich392csd02w) on a separate network is not in the host's own ARP table.
In the simplest case, a network is formed by a number of hosts connected by cables to a hub. Any packet (frame) a host sends across the cable connected to the hub is repeated by the hub to the other cables connected to the hub. If hosts on the same network are connected by a hub, we have a reasonable picture of how things work. However, hubs are not often used these days for an obvious reason: it's extremely inefficient to have every host on the network receive a copy of every packet sent. A better (and far more common) approach is to use a switch.
A hub is considered a Physical Layer device, a hub detects the beginning and end of the Link Layer message from the signal at the Physical Layer and repeats the message on the other connected cables; i.e. the hub does not send background noise. Hubs additionally detect collisions at the Physical Layer and send signals to connected devices to rebroadcast their message.
A switch is smarter than a hub, in that it only forwards packets (frames) to the host they're addressed to, rather than repeating each packet to every host. A switch "learns" the MAC address of each host connected to it, so it knows what MAC address is plugged into what port (physical connection). Each host gets only the traffic addressed to it (plus broadcast traffic), so a switch is much more efficient, and permits higher through put.
A switch is considered a Link Layer device since it processes packets (frames) based on MAC addresses. A switch additionally performs the same set of signal and collision detection that a hub conducts at the Physical Layer.
Routers are special devices that are connected to multiple networks and make decisions on where next to send, or route, the packets they receive. In addition to forwarding packets for others, routers also generate their own packets used to communicate with other routers. Routers communicate using a routing protocol, which is used to maintain accurate routing information for the Internet and to determine where the best next hop is for a packet based on its destination network. All routers must be connected to at least two different networks to actually route packets.
A router is considered a Network Layer device since it processes packets based on IP addresses. A router additionally performs the same functions of a switch (a router reads and writes MAC addresses) and a hub (detects signals and collisions).
Summary. Link Layer addressing compares to Network Layer addressing as switches compare to routers. A switch forwards packets based on MAC Addresses, while routers forward packets based on IP Addresses. Switches learn about host MAC addresses as a result of the network traffic created by each host. Each MAC address is associated with a connection to the switch, or physical port number, and the association is stored in a table in the switch's internal memory.
Sippernet is the colloquial name for the DoD network - isolated from the Internet - that carries IP data that is CONFIDENTIAL or SECRET. SIPRNet packets are encrypted at the Link Layer, allowing CLASSIFIED data to travel across untrusted paths (e.g. from a ship to a military satellite), while also allowing non-secure services to still be used at higher levels of the protocol stack.