# Unix Networking Learn the basics of Unix networking and how to make TCP connections. This material is part of [architecture & deployment course](https://github.com/MediaComem/comem-archidep) for [Media Engineering](https://heig-vd.ch/formations/bachelor/filieres/ingenierie-des-medias). **You will need** * A Unix CLI * An Ubuntu server with a public IP address to connect to **Recommended reading** * [Unix Administration](../unix-admin/) * [Unix Processes](../unix-processes/) --- class: center, middle ## Computer networking .breadcrumbs[<a href="#1">Unix Networking</a>] <img class='w70' src='images/first-internetworked-connection.jpg' /> --- ### OSI model .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a>] The [Open Systems Interconnection (OSI) model][osi] standardizes communications between computing system, allowing interoperability with standard protocols. <p class='center'><img class='w70' src='images/osi.jpg' /></p> A layer serves the layer above it and is served by the layer below it. --- ### TCP/IP model .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a>] The [internet protocol suite][tcp-ip] is the conceptual model used on the Internet and similar computer networks. It is commonly known as TCP/IP since the Transmission Control Protocol (TCP) and the Internet Protocol (IP) are its foundational protocols. <p class='center'><img class='w50' src='images/tcp-ip.png' /></p> It was originally developed for [ARPANET][arpanet]. --- ### OSI vs. TCP/IP .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a>] The OSI and TCP/IP models describe the same technologies, but categorize them a little differently. <p class='center'><img class='w70' src='images/osi-vs-tcp-ip.jpg' /></p> The OSI model is used more as a theoretical construct to reason about networking systems, while the TCP/IP model is more in line with how Internet protocols are designed and used in practice. --- ### IP addressing .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a>] The [Internet Protocol (IP)][ip] is the principal communications protocol of the Internet. It allows delivering packets from a source host to a destination host based solely on IP addresses. It is a **network layer** protocol (OSI layer 3). Version 4 of the protocol ([**IPv4**][ipv4]), in use since 1983, uses a 32-bit address space typically represented in 4 dotted decimal notation, with each octet (8 bits) containing a value between 0 and 255 (i.e. 2⁸ possibilities): ``` `10.199.0.5` ``` Version 6 of the protocol ([**IPv6**][ipv6]) was developed more recently because the world is running out of IPv4 addresses (4 billion IPv4 addresses is not enough in the [Internet of Things (IoT)][iot] world). It's an [internet standard][internet-standard] since 2017. It uses a 128-bit address space typically represented as 8 groups of 4 hexadecimal digits: ``` `2001:0db8:0000:cbad:4321:0000:0000:1234` ``` --- #### IP networks .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#6">IP addressing</a>] Each computer that is publicly accessible on the Internet has a **public IP address**. To facilite routing, IP addresses are logically divided into networks. <p class='center'><img class='w60' src='images/ip-network-vs-host.png' /></p> For example, assuming we use the address `40.127.1.70` and a prefix of 16 bits: * The **network prefix or identifier** would be the first 16 bits: `40.127`. * The **host identifier** would be the last 16 bits: `1.70`. This allows the physical routing devices that are part of the Internet to direct traffic to the correct geographical area and machine(s). > The [Internet Assigned Numbers Authority (IANA)][iana] is the organization responsible for dividing the Internet itself into global networks, > each in turn administered by regional organizations. > You can find the list of registered networks in the [IPv4 Address Space Registry][iana-ipv4] and [IPv6 Address Space Registry][iana-ipv6]. --- #### Subnetworks .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#6">IP addressing</a>] [Subnetting][subnet] can be used to further improve efficiency in the utilization of the relatively small address space available. <p class='center'><img class='w70' src='images/subnet.png' /></p> Instead of having thousands of computers in the same network all able to directly contact each other, subnetting allows organizations to create smaller, isolated networks with fewer computers. This can be used to define **complex network structures** within an organization or to **improve security**. --- #### Reserved address spaces .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#6">IP addressing</a>] Certain ranges of IP addresses are **reserved for private networks**. In this address space you **cannot communicate with public machines** without a NAT gateway or proxy. There are three reserved ranges in the IPv4 address space: First address | Last address | [Netmask][subnet] | [CIDR][cidr] :--- | :--- | :--- | :--- 10.0.0.0 | 10.255.255.255 | 255.0.0.0 | /8 172.16.0.0 | 172.31.255.255 | 255.240.0.0 | /12 192.168.0.0 | 192.168.255.255 | 255.255.0.0 | /16 Additionally, the following range is **reserved for a virtual network interface**, allowing networking applications running on the same machine to communicate with one another: First address | Last address | [Netmask][subnet] | [CIDR][cidr] :--- | :--- | :--- | :--- 127.0.0.1 | 127.255.255.255 | 255.0.0.0 | /8 --- #### There's no place like 127.0.0.1 .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#6">IP addressing</a>] `localhost` is a hostname that refers to the current computer used to access it. It normally resolves resolves to the IPv4 loopback address `127.0.0.1`, and to the IPv6 loopback address `::1`. When you or a program makes a request to `localhost` or `127.0.0.1`, you are contacting your own computer, bypassing network hardware but otherwise behaving the same way as any other network call. <p class='center'><img class='w50' src='images/localhost.jpg' /></p> --- #### 0.0.0.0 is everyone .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#6">IP addressing</a>] You will sometimes encounter [`0.0.0.0`][0000]. This is not an actual IP address. One computer can have several IP addresses. Processes that listen for incoming requests (e.g. a database or a web server) generally allow you to **restrict which IP address they can be reached on**. You may only want to accept requests to one specific address. When you want to allow anyone to reach the process on any IP address the computer may have, you can sometimes use **`0.0.0.0`** as a special notation that means "**all IP addresses on the local machine**". The IPv6 equivalent is `::`. --- #### Network address translation .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#6">IP addressing</a>] [Network address translation (NAT)][nat] is a method of **remapping one IP address space into another** as traffic goes through a routing device. .grid-40[ It is very commonly used for **IP masquerading**, a technique that hides an entire IP address space (such as private IP addresses) behind a single public IP address. The router typically translates the private IP addresses of computers in an organization's network into a single public IP address assigned to the organization. ] .grid-60[ <img class='w100' src='images/nat.jpg' /> ] .container[ Other computers on the Internet see the traffic as originating from the routing device with the public IP address instead of the hidden computer in the private network. This technique helps conserve IPv4 address space. ] --- ### Ports .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a>] In computer networking, a port is an **endpoint of communication** associated with an IP address and protocol type. The most commonly used protocols that use ports are the [Transmission Control Protocol (TCP)][tcp] and the [User Datagram Protocol (UDP)][udp], which are **transport layer** protocols (OSI layer 4). A port is represented as an unsigned 16-bit number, from 0 to 65,535 (2¹⁶ - 1). A port number is always associated with an IP address and the type of transport protocol used for communication. For example, when a browser displays a web page, it is generally making a TCP connection to an IP address on port 80. You can see this information if you access a web page with a command-line HTTP client like [cURL][curl]: ```bash $> curl -v http://google.com/ ... ** Connected to google.com (216.58.215.238) port 80 (#0) ... ``` --- #### Multiplexing .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#13">Ports</a>] .grid-45[ A typical computer only has one IP address. However, one client can **open many connections at the same time to a given IP address and server port** (up to 65535, one for each source port). A client can also open multiple connections to **different ports** at the same time. ] .grid-55[ <img class='w100' src='images/ports.jpg' /> ] .container[ For example, a client may open 4 simultaneous TCP connections to a server: * On port 22 to connect with an SSH client. * On port 25 to retrieve mails with the SMTP protocol. * On port 80 to request a web page with a browser using the HTTP protocol. * On port 80 (again) to simultaneously retrieve a JavaScript file using the HTTP protocol. ] --- #### Registered port numbers .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#13">Ports</a>] The [Internet Assigned Numbers Authority (IANA)][iana] maintains a list of the [official assignments of port numbers][iana-ports] for specific uses, although this is not always respected in practice. Here are some of the most common: Port | Use :--- | :--- 20, 21 | FTP 22 | SSH 25 | SMTP 53 | DNS 80, 443 | HTTP 3306 | MySQL database 5432 | PostgreSQL database 27017 | MongoDB database See the [full list][registered-ports]. --- #### Well-known ports .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#13">Ports</a>] The port numbers in the range from 0 to 1023 are the **well-known ports** or **system ports**. They are used by system processes that provide widely used types of network services, such as SSH or DNS. On Unix-like operating systems, a process must execute with **superuser privileges** to be able to bind a network socket on a well-known port. --- ### Domain name system .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a>] The **Domain Name System (DNS)** is a hierarchical decentralized naming system for computers connected to the Internet or a private network. Most prominently, it **translates human-readable domain names** (like `google.com`) **to numerical IP addresses** (like `40.127.1.70`) needed for locating computers with the underlying network protocols. The Domain Name System has been an essential component of the functionality of the Internet since 1985. <p class='center'><img class='w70' src='images/dns.jpg' /></p> --- #### DNS hierarchy .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#2">Computer networking</a> > <a href="#17">Domain name system</a>] The [Internet Corporation for Assigned Names and Numbers (ICANN)][icann] is responsible for managing [top-level domains (TLDs)][tld] like `.com`. Management of second-level domains and so on is delegated to other organizations. <p class='center'><img class='w80' src='images/dns-hierarchy.png' /></p> You can buy your own [generic top-level domain][gtld] since 2012 for $185,000. --- class: center, middle ## Unix networking .breadcrumbs[<a href="#1">Unix Networking</a>] <img class='w50' src='images/unix.png' /> > Useful commands for unix networking. --- ### The `ip` command .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#19">Unix networking</a>] The **`ip`** command is used to manipulate and display IP network information. Type `ip address` to check your computer's IP address(es). ```bash $> ip address 1: `lo`: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue ... link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet `127.0.0.1`/8 scope host lo valid_lft forever preferred_lft forever inet6 `::1`/128 scope host valid_lft forever preferred_lft forever 2: `eth0`: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 ... link/ether 06:5f:44:85:36:92 brd ff:ff:ff:ff:ff:ff inet `172.31.39.219`/20 brd 172.31.47.255 scope global dynamic eth0 valid_lft 2665sec preferred_lft 2665sec inet6 `fe80::45f:44ff:fe85:3692`/64 scope link valid_lft forever preferred_lft forever ``` In this sample output, there are **two network interfaces**: * The [virtual loopback interface][loopback] (`lo`) through which applications can communicate on the computer itself without actually hitting the network. * A physical Ethernet interface (`eth0`), which has the private IP address `172.31.39.219`. This is the computer's address in its local network. --- ### The `ping` command .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#19">Unix networking</a>] The [`ping` command][ping] tests the reachability of a host on an IP network. It measures the round-trip time for messages sent to a computer and echoed back. The name comes from [active sonar][ping-sonar] terminology that sends a pulse of sound and listens for the echo to detect objects under water. It uses the [Internet Control Message Protocol (ICMP)][icmp], a **network layer** protocol (OSI layer 3). ```bash $> ping -c 1 google.com PING `google.com` (`172.217.21.238`) 56(84) bytes of data. 64 bytes from example.net (172.217.21.238): icmp_seq=1 ttl=53 time=`1.12 ms` --- google.com ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 1.125/1.125/1.125/0.000 ms ``` In this example, you can see that the domain name `google.com` was translated to the public IP address `172.217.21.238` by the Domain Name System, and that the round-trip to that computer took 1.12 milliseconds. > Remove the `-c 1` option to keep pinging once per second. --- ### The `traceroute` command .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#19">Unix networking</a>] The [`traceroute` command][traceroute] displays the path and transit delays of packets across an IP network. ```bash $> traceroute google.com traceroute to google.com (172.217.22.78), 30 hops max, 60 byte packets 1 example.amazonaws.com (54.93.0.48) 18.214 ms ... 2 100.66.0.184 (100.66.0.184) 13.718 ms ... 3 100.66.0.91 (100.66.0.91) 25.227 ms ... ... 7 52.93.111.27 (52.93.111.27) 18.485 ms ... 8 74.125.49.104 (74.125.49.104) 0.740 ms ... 9 108.170.251.129 (108.170.251.129) 1.636 ms ... 10 72.14.232.35 (72.14.232.35) 0.689 ms ... 11 example.net (172.217.22.78) 0.583 ms ... ``` .grid-50[ Router addresses can be superimposed upon maps of their physical locations. This example shows a request from New Zealand to an IP in Massachusetts which takes a route that passes through Europe. ] .grid-50[ <img class='w100' src='images/traceroute.png' /> ] --- ### The `mtr` command .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#19">Unix networking</a>] The `mtr` command, originally named **M**att's **tr**aceroute, combines the `ping` and `traceroute` utilities into a single network diagnostic tool. ```bash $> mtr google.com My traceroute [v0.92] ip-172-31-39-219 (172.31.39.219) 2018-12-05T15:55:42+0000 Host Loss% Snt Last Avg Best Wrst StDev 1. ??? 2. ??? 3. ??? 4. 100.65.1.225 0.0% 13 0.4 0.5 0.4 1.1 0.2 5. 52.93.23.133 0.0% 13 3.9 1.8 1.4 3.9 0.8 6. 54.239.107.138 0.0% 13 1.9 2.7 1.8 7.2 1.7 7. 52.93.111.11 0.0% 13 1.6 2.3 1.4 9.9 2.3 8. 209.85.149.182 0.0% 13 3.2 3.9 1.5 22.5 5.8 9. 108.170.252.1 0.0% 13 2.4 2.3 2.1 3.0 0.2 10. 66.249.95.169 0.0% 13 2.2 2.2 1.8 3.4 0.5 11. example.net 0.0% 13 1.2 1.2 1.2 1.5 0.1 ``` --- ### The `ss` command .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#19">Unix networking</a>] The [**s**ocket **s**tatistics (`ss`) command][ss] (or the older `netstat` command) displays information about the open [**network sockets**][socket] on the computer. > A socket is the software representation of a network communication's endpoint. > For a TCP connection in an IP network, it corresponds to a connection made on an IP address and port number. ```bash $> ss -tlpn State Recv-Q Send-Q Local Address:Port Peer Address:Port LISTEN 0 80 127.0.0.1:3306 0.0.0.0:* mysqld... LISTEN 0 128 127.0.0.53%lo:53 0.0.0.0:* systemd-resolve... LISTEN 0 128 0.0.0.0:22 0.0.0.0:* sshd... LISTEN 0 128 [::]:22 [::]:* sshd... ``` The above command shows which processes are listening for TCP connections. In this example, we can see that there is a MySQL database listening on port 3306, a DNS resolver on port 53, and an SSH server on port 22. --- ### The `nc` command .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#19">Unix networking</a>] The [**n**et**c**at (`nc`) command][nc] can read from and write to network connections using TCP or UDP. ```bash $> nc -zv -w 2 google.com 80 Connection to google.com 80 port [tcp/http] succeeded! $> nc -zv -w 2 google.com 81 nc: connect to google.com port 81 (tcp) timed out: Operation now in progress nc: connect to google.com port 81 (tcp) failed: Network is unreachable ``` For example, the above 2 commands check whether port 80 and 81 are open on the computer reached by resolving the domain name `google.com`. --- class: center, middle ## Making connections .breadcrumbs[<a href="#1">Unix Networking</a>] <p class='center'><img class='w50' src='images/connected-computers.png' /></p> --- ### Raw TCP connection .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a>] Assuming Bob's server has the public IP address `W.X.Y.Z` and Alice's server has the public IP address `S.T.U.V`. .grid-50[ Bob should use `nc` to listen for TCP connections on port 3000: ```bash $> nc -l 3000 ``` If Bob types some text and presses Enter to send it, it should be displayed in Alice's terminal. ```bash $> nc -l 3000 *Hello World ``` ] .grid-50[ Alice should use `nc` to connect to TCP port 3000 on Bob's server: ```bash $> nc W.X.Y.Z 3000 ``` Similarly, if Alice types and sends some text, it should appear in Bob's terminal: ```bash $> nc W.X.Y.Z 3000 Hello *World ``` ] .container[ You have a two-way TCP connection running. ] --- ### HTTP request .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a>] Playing with TCP is all well and good, but it's a little low level. Let's do something that your browser does every day: an HTTP request. Find out Google's IP address (`O.P.Q.R` in this example): ```bash $> ping -c 1 google.com PING google.com (`O.P.Q.R`) 56(84) bytes of data. 64 bytes from example.net (O.P.Q.R): icmp_seq=1 ttl=53 time=0.890 ms ... ``` As a reminder, a basic [HTTP request][http-req] looks like this: ``` GET /path HTTP/1.1 Header1: Value1 Header2: Value2 ``` The first line, the status line, indicates the [HTTP method][http-methods], resource path and protocol version. The next lines are [headers][http-headers] to send additional parameters to the server. --- #### Making an HTTP request to Google .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a> > <a href="#28">HTTP request</a>] Open a TCP connection to the Google IP address you previously found: ```bash $> nc O.P.Q.R 80 ``` Type the following lines exactly, followed by **two new lines** to terminate the message: ``` GET / HTTP/1.1 Host: google.com ``` You have just sent an HTTP request to one of Google's servers, asking for the website `google.com` with the `Host` header. It should respond with something like this: ``` HTTP/1.1 301 Moved Permanently Location: http://www.google.com/ ... ``` By responding with the HTTP status code [301 Moved Permanently][http-301], Google's server is telling you that the website `google.com` has permanently moved to `www.google.com`. --- #### Following Google's redirect .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a> > <a href="#28">HTTP request</a>] The connection should still be open. You can make another request, this time to `www.google.com`, by typing the following lines exactly, again followed by **two new lines**: ``` GET / HTTP/1.1 Host: www.google.com ``` This time the server should send you Google's home page, in HTML format as indicated by the `Content-Type` header: ``` HTTP/1.1 200 OK Date: Wed, 05 Dec 2018 17:33:14 GMT Content-Type: text/html; charset=ISO-8859-1 ... <!doctype html> <html itemscope="" itemtype="http://schema.org/WebPage" lang="de"> ... </html> ``` --- ### HTTP response .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a>] Let's make a real-world request with an actual browser this time. Listen for TCP connections on your server on port 3000: ```bash $> nc -l 3000 ``` --- #### Making an HTTP request with a browser .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a> > <a href="#31">HTTP response</a>] Assuming that your server has the public IP address `K.L.M.N`, visit `http://K.L.M.N:3000` in your browser. Your browser will make an HTTP request to the computer at IP address `K.L.M.N` on port `3000`. You should see the request in the terminal where netcat is running: ``` GET / HTTP/1.1 Host: 3.121.150.168:3000 Connection: keep-alive Cache-Control: max-age=0 Upgrade-Insecure-Requests: 1 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.110 Safari/537.36 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8 Accept-Encoding: gzip, deflate Accept-Language: en-US,en;q=0.9,fr;q=0.8,pt;q=0.7 ``` --- #### Sending a plain text HTTP response .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a> > <a href="#31">HTTP response</a>] Type or copy-paste the following text into the terminal: ``` HTTP/1.1 200 OK Content-Type: text/plain Hello World ``` Stop netcat with Ctrl-C. The client should display the text once the connection has been closed. --- #### Sending an HTTP response with HTML .breadcrumbs[<a href="#1">Unix Networking</a> > <a href="#26">Making connections</a> > <a href="#31">HTTP response</a>] Re-run the netcat command on the server: ```bash $> nc -l 3000 ``` Re-visit `http://K.L.M.N:3000` in your browser, then type or copy-paste the following text into the terminal once you have received the request: ``` HTTP/1.1 200 OK Content-Type: text/html <!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title>Some HTML</title> <style>body { color: red; }</style> </head> <body> <h1>This is HTML</h1> </body> </html> ``` Stop netcat with Ctrl-C. You should see the HTML text appear in your browser. --- ## References .breadcrumbs[<a href="#1">Unix Networking</a>] * [Internet Protocol][ip] * [IP address](https://en.wikipedia.org/wiki/IP_address) * [IPv4][ipv4] & [IPv6][ipv6] * [Subnetworks][subnet] * [Network Address Translation (NAT)][nat] * [Ports][port] * [List of TCP and UDP port numbers][registered-ports] * [Ops School Curriculum](http://www.opsschool.org) * [Networking 101](http://www.opsschool.org/networking_101.html) * [Networking 202](http://www.opsschool.org/networking_201.html) * [Unix Networking Basics for the Beginner](https://www.networkworld.com/article/2693416/unix-networking-basics-for-the-beginner.html) * [Unix Top Networking Commands and What They Tell You](https://www.networkworld.com/article/2697039/unix-top-networking-commands-and-what-they-tell-you.html) * [What happens when you type google.com into your browser and press enter?](https://github.com/alex/what-happens-when) [0000]: https://en.wikipedia.org/wiki/0.0.0.0 [arpanet]: https://en.wikipedia.org/wiki/ARPANET [cidr]: https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing [curl]: https://curl.haxx.se [dns]: https://en.wikipedia.org/wiki/Domain_Name_System [gtld]: https://en.wikipedia.org/wiki/Generic_top-level_domain [http-301]: https://httpstatuses.com/301 [http-headers]: https://en.wikipedia.org/wiki/List_of_HTTP_header_fields [http-methods]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods [http-req]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages#HTTP_Requests [http-res]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages#HTTP_Responses [iana]: https://www.iana.org [iana-ipv4]: https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtml [iana-ipv6]: https://www.iana.org/assignments/ipv6-address-space/ipv6-address-space.xhtml [iana-ports]: https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml [icann]: https://en.wikipedia.org/wiki/ICANN [icmp]: https://en.wikipedia.org/wiki/Internet_Control_Message_Protocol [internet-standard]: https://en.wikipedia.org/wiki/Internet_Standard [iot]: https://en.wikipedia.org/wiki/Internet_of_things [ip]: https://en.wikipedia.org/wiki/Internet_Protocol [ipv4]: https://en.wikipedia.org/wiki/IPv4 [ipv6]: https://en.wikipedia.org/wiki/IPv6 [loopback]: https://en.wikipedia.org/wiki/Loopback#Virtual_loopback_interface [mtr]: https://en.wikipedia.org/wiki/MTR_(software) [nat]: https://en.wikipedia.org/wiki/Network_address_translation [nc]: https://en.wikipedia.org/wiki/Netcat [osi]: https://en.wikipedia.org/wiki/OSI_model [ping]: https://en.wikipedia.org/wiki/Ping_(networking_utility) [ping-sonar]: https://en.wikipedia.org/wiki/Sonar#Active_sonar [port]: https://en.wikipedia.org/wiki/Port_(computer_networking) [registered-ports]: https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers [socket]: https://en.wikipedia.org/wiki/Network_socket [ss]: http://man7.org/linux/man-pages/man8/ss.8.html [subnet]: https://en.wikipedia.org/wiki/Subnetwork [tcp]: https://en.wikipedia.org/wiki/Transmission_Control_Protocol [tcp-ip]: https://en.wikipedia.org/wiki/Internet_protocol_suite [tld]: https://en.wikipedia.org/wiki/Top-level_domain [traceroute]: https://en.wikipedia.org/wiki/Traceroute [udp]: https://en.wikipedia.org/wiki/User_Datagram_Protocol