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# 24 Network Hardware

In this chapter we’ll take a look at a number of hardware networking components.

Some of these you have at home or on your computer already.

## 24.1 Terminology and Components

- Network Topology: describes the layout of a network, how devices and nodes are connected, and how data flows from one part of the network to another.

- Mbs: Megabits per second (compare to “MBs”, megabytes per second).

- Gbs: Gigabits per second, 1000 Mbs (compare to “GBs”, gigabytes per second).

- Bandwidth: Measured in bits per second, how much data a certain type of cable can carry over a certain amount of time. e.g. 500 Mbs.

- ISP: Internet Service Provider, the company that you pay for your Internet connection. (Or that provides it, even if you aren’t paying!)

- Twisted Pair Cable: What people typically think of as “Ethernet cable”. It’s a cable with plug-in jacks on either end. Internally, pairs of wires are twisted together to reduce interference. These cables have published maximum length “runs” that determine how far you can string a cable before you run into trouble, usually 50-100 meters depending on the cable and traffic speed.

  - 10baseT: 10 Mbs twisted-pair for Ethernet
  - 100baseTX: 100 Mbs twisted-pair for Fast Ethernet.
  - 1000baseT: 1 Gbs twisted-pair for Gigabit Ethernet.
  - 10GbaseT: 10 Gbs twisted-pair for 10 Gigabit Ethernet.
  - Category-5: Called cat-5 for short, a twisted pair wire built to category-5 specs. Good for Fast Ethernet.
  - Category-6: Called cat-6 for short, a twisted pair wire built to category-6 specs. Good for Gigabit Ethernet.

- Network Port: Not to be confused with TCP or UDP port numbers, which are completely different, in this context refers to a physical socket on a device where you can plug in a network cable.

- Ethernet Cable: Conversational term meaning a twisted pair cable that you plug into Ethernet devices.

- Crossover Cable: Cable where the transmit and receive pins are swapped on one end of the cable. This is typically used when connecting two devices directly to each other when normally a switch or hub would be used as an intermediary. If you’re plugging one computer directly into another with an Ethernet cable, it should probably be a crossover cable. Opposite of straight-through cable.

- Auto-sensing: A network port that can tell if it has a straight-through or cross-over cable plugged into it, and can reverse the transmit and receive signals if it has to.

- Thin-net/Thick-net: Obsolete coaxial cabling used for Ethernet.

- LAN: Local Area Network. For Ethernet, this would be the network at your house. Think of a single IP subnet.

- WAN: Wide Area Network. Network that’s not LAN. Think of a collection of LANs on University or company campus.

- Dynamic IP: This is when your IP gets set automatically with DHCP, for example. Your IP address might change over time.

- Static IP: This is when you hardcode the IP address for a certain device. The IP address doesn’t change until you actively enter a new one.

- Network Interface Controller (NIC), Network Interface Card, Network Adapter, LAN Adapter, Network Card: A bunch of different names for a hardware device that allows a computer to get on the network. This card might have Ethernet ports on it, or maybe it’s just pure wireless. It’s probably not even be a proper card these days–maybe it’s all on-board the same chip as a bunch of other I/O devices built into the motherboard.

- Network Device (OS): A software device structure in the operating system that typically maps to a NIC. Some network devices like the loopback device don’t actually use a physical piece of hardware and just “transmit” data internally within the OS. On Unix-likes, these devices are named things like `eth0`, `en1`, `wlan2` and so on. The loopback device is typically called `lo`.

- MAC address: Media Access Control address. A unique Link Layer address for computers. With Ethernet, the MAC address is 6 bytes, and is usually written as hex bytes separated by colons: `12:34:56:78:9A:BC`. These address must be unique on a LAN for Ethernet to function correctly.

- Hub: A device that allows you to connect several computers via Ethernet cables. It’ll have 4, 8, or more ports on the front. All these devices plugged into those ports are effectively on the same “wire” once connected, which is to say that any Ethernet packet transmitted is seen by all devices plugged into the HUB. You don’t typically see these any longer, since switches perform the same role better.

- Switch: A hub with some brains. It knows the MAC addresses on the other side of the ports so it doesn’t have to retransmit Ethernet packets to everyone. It just sends them down the proper wire to the correct destination. Help prevent network overload.

- Router: A Network Layer device that have multiple interfaces and chooses the correct one to send traffic down so that it will eventually reach its destination. Routers contain routing tables that let them decide where to forward a packet with a given IP address.

- Default Gateway: A router that handles traffic to all other destinations, if a specific route to the destination isn’t known. A computer’s routing table specifies the default gateway. On a home LAN, this is the IP of the “router” the ISP gave you.

  Imagine an island with a small town on it. The island is connected to the mainland by a single bridge. If someone wants to know where to go in town, you give them directions in town. For all other destinations, they drive across the bridge. In this analogy, the bridge is the default gateway for traffic.

- Broadcast: To send traffic to everyone on the LAN. This can be done at the Link Layer by sending an Ethernet frame to MAC address `ff:ff:ff:ff:ff`, or at the Network Layer by sending an IP packet with all the host bits set to `1`. For instance, if the network is `192.168.0.0/16`, the broadcast address would be `192.168.255.255`. You can also broadcast to `255.255.255.255` for the same effect. IP routers do not forward IP broadcast packets, so they are always restricted to the LAN.

- Wi-Fi: Short for Wireless Fidelity (a non-technical marketing trademark presumably meant to pun Hi-Fi), this is your wireless LAN connection. Speaks Ethernet at the Link Layer. Very similar to using an Ethernet cable, except instead of electricity over copper, it uses radio waves.

- Firewall: A computer or device at or near where your LAN connects to your ISP that filters traffic, preventing unwanted traffic from being transmitted on your LAN. Keeps the bad guys out, hopefully.

- NAT: Network Address Translation. A way to have a private IP subnet behind the NAT device that is not visible to the rest of the Internet. The NAT device translates internal IP addresses and ports to an external IP on the router. This is why if you go to Google and ask “what is my ip”, you’ll get a different number than you’ll see when you look at the settings on your computer. NAT is in the middle, translating between your internal LAN IP address and the external, publicly-visible IP address. We’ll talk more about the details of this mechanism later.

- Private Network IPv4 Addresses: For LANs not connected to the Internet or LANs behind NAT, there are three common subnets that are used: `10.0.0.0/8`, `172.16.0.0/12`, `192.168.0.0/16`. These are reserved for private use; no public sites will ever use them, so you can put them on your LAN without worrying about conflicts.

- WiFi Modem/WiFi Router: Loosely refers to a consumer-grade device that you get when you sign up with an ISP for service. Often does a variety of things

- Rack-mount: If a device doesn’t come in a nice plastic case or isn’t meant to be plugged directly into a computer, it might be rack-mount. These are larger non-consumer devices like routers, switches, or banks of disks, that get stacked up in “racks”.

- Upload: Transferring a file from your local device to a remote device.

- Download: Transferring a file from a remote device to your local device.

- Symmetric: In the context of transfer speeds, means that a connection offers the same speeds in both directions. “1 Gbs symmetric” means that upload and download speeds are 1 Gbs.

- Asymmetric: In the context of transfer speeds, means that a connection offers different speeds in either direction. Usually written as something like “600 Mbs down, 20 Mbs up”, for example, indicating download and upload speeds. Often shortened to “600 by 20” conversationally. Most general usage is people download things, not uploading, so companies that provide service allocate more of their total bandwidth on the download side.

- Cable (from the cable company): Many cable television companies offer Internet connectivity over the coaxial cable line they’ve run to your house. Speeds up to 1 Gbs aren’t unheard of. Typically a neighborhood shares bandwidth, so your speeds will drop in the evening when everyone living around you is watching movies. Most cable offerings are asymmetric.

- DSL: Digital Subscriber Line. Many telephone companies offer Internet connectivity over the phone lines they’ve run to your house. It’s slower than cable at around 100 Mbs, but bandwidth is not shared with neighbors. Most DSL offerings are asymmetric.

- Fiber: Short for optical fiber, uses light through glass “wires” instead of electricity through copper wires. Very quick. Many ISPs that offer relatively-cheap fiber have packages that deliver 1 Gbs symmetric.

- Modem: Short for Modulator/Demodulator, converts signals in one form to signals in another form. Historically, this meant turning sounds transmitted over a telephone landline into data. In modern usage, it means converting the signals on your Ethernet LAN to whatever form is needed by the ISP, e.g. cable or DSL.

- Bridge: A device that connects two networks at the link level, allowing them to behave as a single network. Some bridges blindly forward all traffic, other bridges are smarter about it. Cable modems are a type of bridge, though they often come built into the same box as a router and switch.

- Vampire Tap: Back in the old days, when you wanted to connect a computer to a thicknet cable, you used one of these awesomely-named devices to do it. Included here just for fun.

## 24.2 Reflect

- What’s the difference between a hub and a switch?

- What does a router do?

- Why would a router with only one network connection not make any sense?

- What kind of device do you connect to with your laptop where you live? Do you use a physical cable to connect?

- No need to write anything for this reflection point unless you’re inclined, but ponder what life was like with 300 bps modems. The author’s first modem was a VICMODEM, literally two million times slower than a modern cable connection. That was 40 years ago. Now imagine network speeds in the year 2062.

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