Networking and TCP/IP

An important function of a system administrator is to set up, configure, and monitor a network. This may involve planning, configuring, and connecting the devices on a local area network, to planning and implementing a large network that interfaces with an outside network, and to monitoring networks for various sorts of attacks, such as denial of service attacks.

In order to prepare for this type of work, we need at least a basic understanding of how the internet works and how local devices interact with the internet. In this section, we will focus mostly on internet addressing, but we will also devote some space to TCP and UDP, two protocols for transmitting data.

Connecting two or more devices together nowadays involves the TCP/IP or the UDP/IP protocols, otherwise part of the Internet protocol suite. This suite is a kind of expression of the more generalized OSI communication model.

The internet protocol suite is generally framed as a series of layers beginning with a lower layer, the link layer, that interfaces with internet capable hardware, to the highest layer, the application layer.

The link layer describes the local area network. Devices connected locally, e.g., via Ethernet cables, comprise the link layer. The link layer connects to the internet layer. Data going into or out of a local network must be negotiated between these two layers.

The internet layer makes the internet possible by basically making the ability to transmit data among multiple networks possible. (The internet is, in fact, a network of networks). The primary characteristic of the internet layer is the IP address, which currently comes in two versions: IPv4 (32 bit) and IPv6 (128 bit). IP addresses are used to locate hosts on a network.

The transport layer makes the exchange of data on the internet possible. There are two dominant protocols attached to this layer: UDP and TCP. Very generally, UDP is used when the integrity of data is less important than the its ability to reach its destination. For example, streaming video, VOIP, and online gaming are often transported via UDP because the loss of some pixels or some audio is acceptable. TCP is used when the integrity of the data is important. If the data cannot be transmitted without error, then the data won't reach its final destination until the error is corrected.

The application layer provides the ability to use the internet in particular ways. For example, the HTTP protocol enables the web, which is simply an application on the internet. The SMTP, IMAP, and POP protocols control email exchange. DNS is a system that maps IP addresses to domain names. In this book, we use SSH, also part of the application layer, to connect to remote computers.

By application, they simply mean that these protocols provide the functionality for applications. They are not themselves considered user applications, like a web browser.

The Internet Protocol Suite

ARP (Address Resolution Protocol)

ARP (Address Resolution Protocol) is a protocol at the link layer and is used to map network addresses, like an IP address, to the ethernet addresses, also called the MAC or Media Access Control address, or the hardware address. Routers use MAC addresses to enable communication inside networks (w/in subnets or local area networks) so that computers within a local network can talk to each other. Networks are designed so that IP addresses are associated with MAC addresses before systems can communicate over a network. Everyone of your internet capable devices, your smartphone, your laptop, your internet connected toaster, have a MAC address.

To get ARP info for a system, we use the ip command, which uses regular options (like -b) and names specific objects. To get the MAC address for a specific computer, we can use the following command, where ip is the command and a or link are considered objects (see man ip for details):

ip a

On my home system, the above command produces three numbered sections of output. The first section refers to the lo or loopback device. This is a special device that allows the computer to communicate with itself. It always has an IPv4 address of The next section on my home machine refers to the ethernet card. Currently, I'm connected via wifi, and so this section reports the MAC address for that ethernet card plus some other other information, such as whether the device is down or up. Since there's no physical cable connecting my machine to the router, this section reports DOWN. The third section on my home system refers to the wifi card. Since this is UP (or active), it reports the internal IP address (e.g.,, plus the MAC address, and other details. The internal address is different from the machine's external address, which might be something like

We can get just the link object information with the following command:

ip link

The following two commands help identify parts of the local network (or subnet) and the routing table.

ip neigh
ip route

The ip neigh command produces the ARP cache, basically what other systems your system is aware of on the local network. The ip route command is used to define how data is routed on the network but can also define the routing table. Both of these commands are more commonly used on Linux-based routers.

These details enable the following scenario: A router gets configured to use a specific network address when it's brought online. It searches the sub network for connected MAC addresses that are assigned to wireless cards or ethernet cards. It then assigns each of those MAC addresses an available IP address based on the network address.

Internet Layer

IP (Internet Protocol)

The Internet Protocol, or IP, address is used to uniquely identify a host on a network and place that host at a specific location (its IP address). If that network is subnetted (i.e., routed), then a host's IP address will have a subnet or private IP address that will not be directly exposed to the Internet. Remember this, there are public IP addresses that are distinct from private IP addresses. Public IP addresses are accessible on the internet. Private IP addresses are not, but they are accessible on subnets or local area networks.

Private IP address ranges are reserved address ranges, which means no public internet device will have an IP address within these ranges. The private address ranges include:

Start AddressEnd Address

If you have a router at home, and look at the IP address for at any of your devices connected to that router, like your phone or computer, you will see that it will have an address within one of the ranges above. For example, it might have an IP address beginning with 192.168.X.X. This is a standard IP address range for a home router. The 10.X.X.X private range can assign many more IP addresses on its network, which is why you'll see that IP range on bigger networks, like a university's network. We'll talk more about subnetwork sizes, shortly.

Example Private IP Usage

Let's say my office computer's IP address is via a wired connection. My office neighbor has an IP address of via their wired connection. Both IP addresses are private because they fall within the to range. And it's likely they both exist on the same subnet since they share the first three octets: 10.163.34.XX.

However, if we both, using our respective wired connected computers, searched Google for what's my IP address, we will see that we share the same public IP address, which will be something like That is a public IP address because it does not fall within the ranges listed above.

Without any additional information, therefore, we know that all traffic coming from our computers and going out to the internet looks like it's coming from the same IP address ( And in reverse, all traffic coming from outside our network first goes to before it's routed to our respective computers via the router.

Let's say I also have a laptop in my office, and that it has a wireless connection. When I check with ip a, I find that the laptop had the IP address You can see there's a different pattern with this IP address. The reason it has a different pattern is because this laptop is on an different subnet even though it's physically sitting next to the wired computer. This wireless subnet was configured to allow more hosts to connect to it since it must allow for more devices (i.e., laptops, phones, etc). When I searched Google for my IP address from this laptop, it reports, indicating that UK owns a range of public IP address spaces.

Here's kind of visual diagram of what this network looks like:

Network diagram
Fig. 1. This figure contains a network switch, which is used to route traffic within a subnet. The switch relies solely on MAC addresses and not IP addresses to determine the location of devices on its subnet. The router acts as the interface between the private network and the public network and is managing two subnets: a wired and a wireless one.

Using the ip Command

The ip command can do more than provide us information about our network. We can also use it to turn a connection to the network on or off (and more). Here we disable and then enable a connection on a machine. Note that enp0s3 is the name of my network card/device. Yours might have a different name.

sudo ip link set enp0s3 down
sudo ip link set enp0s3 up

Transport Layer

The internet layer does not transmit content, like web pages or video streams. This is the work of the transport layer. As discussed previously, the two most common transport layer protocols are TCP and UDP.

TCP, Transmission Control Protocol

TCP or Transmission Control Protocol is responsible for the transmission of data and for making sure the data arrives at its destination w/o errors. If there are errors, the data is re-transmitted or halted in case of some failure. Much of the data sent over the internet is sent using TCP.

UDP, User Datagram Protocol

The UDP or User Datagram Protocol performs a similar function as TCP, but it does not error check and data may get lost. UDP is useful for conducting voice over internet calls or for streaming video, such as through YouTube, which uses a type of UDP transmission called QUIC that has builtin encryption.

TCP and UDP Headers

The above protocols send data in data TCP packets or UDP datagrams, but these terms may be used interchangeably. Packets for both protocols include header information to help route the data across the internet. TCP includes ten fields of header data, and UDP includes four fields.

We can see this header data using the tcpdump command, which requires sudo or being root to use. The first part of the IP header contains the source address, then comes the destination address, and so forth. Aside from a few other parts, this is the primary information in an IP header.

You should use tcpdump on your local computer and not on your gcloud instance. First we identify the IP number of a host, which we can do with the ping command, and then run tcpdump:

ping -c1
sudo tcpdump host

While that's running, we can type that IP address in our web browser, or enter, and watch the output of tcpdump.

TCP headers include port information and other mandatory fields for both source and destination servers. The SYN, or synchronize, message is sent when a source or client requests a connection. The ACK, or acknowledgment, message is sent in response, along with a SYN message, to acknowledge the request for a connection. Then the client responds with an additional ACK message. This is referred to as the TCP three-way handshake. In addition to the header info, TCP and UDP packets include the data that's being sent (e.g., a webpage) and error checking if it's TCP.


TCP and UDP connections use ports to bind internet traffic to specific IP addresses. Specifically, a port associates a process with an application (and is part of the application layer of the internet suite), such as a web service or outgoing email. That is, ports provide a way to distinguish and filter internet traffic (web, email, etc) through an IP address. For example, all traffic going to IP address means that this is http traffic for the http web service, since HTTP is commonly associated with port 80. Note that the port info is attached to the end of the IP address via a colon.

Common ports include:

  • 21: FTP
  • 22: SSH
  • 25: SMTP
  • 53: DNS
  • 143: IMAP
  • 443: HTTPS
  • 587: SMTP Secure
  • 993: IMAP Secure

There's a complete list of the 318 default ports/protocols on your Linux systems. It's located in the following file:

less /etc/services

And to get a count of the ports, we can invert grep for lines starting with a pound sign or are empty

grep -Ev "^#|^$" /etc/services | wc -l

See also the Wikipedia page: List of TCP and UDP port numbers

IP Subnetting

Let's now return to the internet layer and discuss one of the major duties of a systems administrator: subnetting.

Subnets are used to carve out smaller and more manageable subnetworks out of a larger network. They are created using routers that have this capability (e.g., commercial use routers) and certain types of network switches.

Private IP Ranges

When subnetting local area networks, we work with the private IP ranges:

Start AddressEnd Address

It's important to be able to work with IP addresses like those listed above in order to subnet; and therefore, we will need to learn a bit of IP math along the way.

IP Meaning

An IPv4 address is 32 bits (8 x 4), or four bytes, in size. In human readable context, it's usually expressed in the following, decimal-based, notation style:


Each set of numbers separated by a dot is referred to as an octet. An octet is a group of 8 bits. Eight bits equal a single byte. By implication, 8 gigabits equals 1 gigabyte, and 8 megabits equals 1 megabyte. We use these symbols to note the terms:


Each bit is represented by either a 1 or a 0. For example, the first address above in binary is:

  • 11000000.10101000.00000001.00000110 is


ByteDecimal Value

IP Math

When doing IP math, one easy way to do it is to simply remember that each bit in each of the above bytes is a placeholder for the following values:

128 64 32 16 8 4 2 1

Alternatively, from low to high:


In binary, 192 is equal to 11000000. It's helpful to work backward. For IP addresses, all octets are 255 or less (256 total, from 0 to 255) and therefore do not exceed 8 bits or places. To convert the integer 192 to binary:

1 * 2^7 = 128
1 * 2^6 =  64 (128 + 64 = 192)

Then STOP. There are no values left, and so the rest are zeroes. Thus: 11000000

Our everyday counting system is base-10, but binary is base-2, and thus another way to convert binary to decimal is to multiple each bit (1 or 0) by the power of base two of its placeholder:

(0 * 2^0) = 0 +
(0 * 2^1) = 0 +
(0 * 2^2) = 0 +
(0 * 2^3) = 0 +
(0 * 2^4) = 0 +
(0 * 2^5) = 0 +
(1 * 2^6) = 64 +
(1 * 2^7) = 128 = 192

Another way to convert to binary: simply subtract the numbers from each value. As long as there is something remaining or the placeholder equals the remainder of the previous subtraction, then the bit equals 1. So:

  • 192 - 128 = 64 -- therefore the first bit is equal to 1.
  • Now take the leftover and subtract it:
  • 64 - 64 = 0 -- therefore the second bit is equal to 1.

Since there is nothing remaining, the rest of the bits equal 0.

Subnetting Examples

Subnetting involves dividing a network into two or more subnets. When we subnet, we first identify the number of hosts, aka, the size, we will require on the subnet. For starters, let's assume that we need a subnet that can assign at most 254 IP addresses to the devices attached to it via the router.

In order to do this, we need two additional IP addresses: the subnet mask and the network address/ID. The network address identifies the network and the subnet mask marks the boundary between the network and the hosts. Knowing or determining the subnet mask allows us to determine how many hosts can exist on a network. Both the network address and the subnet mask can be written as IP addresses, but these IP addresses cannot be assigned to computers on a network.

When we have determined these IPs, we will know the broadcast address. This is the last IP address in a subnet range, and it also cannot be assigned to a connected device/host. The broadcast address is used by a router to communicate to all connected devices on the subnet.

For our sake, let's work through this process backwards; that is, we want to identify and describe a network that we are connected to. Let's work with two example private IP addresses that exist on two separate subnets.

Example IP Address 1:

Using the private IP address, let's derive the network mask and the network address (or ID) from this IP address. First, convert the decimal notation to binary. State the mask, which is /24, or And then derive the network addressing using an bitwise logical AND operation:

11000000.10101000.00000001.00000110 IP    
11111111.11111111.11111111.00000000 Mask  
11000000.10101000.00000001.00000000 Network Address

Note the mask has 24 ones followed by 8 zeroes. That 24 is used as CIDR notation:


For Example 1, we thus have the following subnet information:

Network ID192.168.1.0
Start Range192.168.1.1
End Range192.168.1.254

Example IP Address 2:

For example 2, let's start off with a private IP address of and a mask of /24:

00001010.10100000.00100110.01001011 IP     
11111111.11111111.11111111.00000000 Mask  
00001010.10100000.00100110.00000000 Network Address
Network ID10.160.38.0
Start Range10.160.38.1
End Range10.160.38.254

Example IP Address 3:

For example 3, let's start off with a private IP address of and a mask of /24:

10101100 00010000 00000001 00100111 IP      
11111111 11111111 11111111 00000000 Mask  
10101100 00010000 00000001 00000000 Network Address
Network ID172.16.1.0
Start Range172.16.1.1
End Range172.16.1.254

Determine the Number of Hosts

To determine the number of hosts on a CIDR /24 subnet, we look at the start and end ranges. In all three of the above examples, the start range begins with X.X.X.1 and ends with X.X.X.254. Therefore, there are 254 maximum hosts allowed on these subnets because 1 to 254, inclusive of 1 and 254, is 254.

Example IP Address 4:

The first three examples show instances where the CIDR is set to /24. This only allows 254 maximum hosts on a subnet. If the CIDR is set to /16, then we can theoretically allow 65,534 hosts on a subnet.

For example 4, let's start off then with a private IP address of and a mask of /16:

00001010.00000000.00000101.00010111 IP Address:
11111111.11111111.00000000.00000000 Mask:
00001010.00000000.00000000.00000000 Network ID:
IP Address10.0.5.23
Network ID10.0.0.0
Start Range10.0.0.1
End Range10.0.255.254

Since the last two octets/bytes now vary, we count up by each octet. Therefore, the number of hosts is:

IPs 256 256
  • Number of Hosts = 256 x 256 = 65536
  • Subtract Network ID (1) and Broadcast (1) = 2 IP addresses
  • Number of Usable Hosts = 256 x 256 - 2 = 65534

IPv6 subnetting

We're not going to cover IPv6 subnetting, but if you're interested, this is a nice article: IPv6 subnetting overview


As a systems administrator, it's important to have a basic understanding of how networking works, and the basic models used to describe the internet and its applications. System administrators have to know how to create subnets and defend against various network-based attacks.

In order to acquire a basic understanding, this section covered topics that included:

  • the internet protocol suite
    • link layer
    • internet layer
    • transport layer
  • IP subnetting
    • private IP ranges
    • IP math

In the next section, we extend upon this and discuss the domain name system (DNS) and domain names.