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After published a post How To Become A Hacker - Guide For Beginners, there have been some questions regarding networking skills, It is very important topic to become a hacker, so this article will explain the basic networking a Ethical hacker or Penetration tester must learn.


Here is a list of basics:

1. DHCP
2. NAT
3. Subnetting
4. IPv4
5. IPv6
6. Public vs Private IP
7. DNS
8. Routers and switches
9. VLANs
10. OSI model
11. MAC addressing
12. ARP

1. DHCP

• Protocols

• Clients and servers

• Permission slips

• How DHCP works

1. Your go on your computer to connect to the Internet.
2. The network requests an IP address (this is actually referred to as a DHCP discover message).
3. On behalf of your computers request, the DHCP server allocates (leases) to your computer an IP address. This is referred to as the DHCP offer message.
4. Your computer (remember—youre the DHCP client) takes the first IP address offer that comes along. It then responds with a DHCP request message that verifies the IP address thats been offered and accepted.
5. DHCP then updates the appropriate network servers with the IP address and other configuration information for your computer.
6. Your computer (or whatever network device youre using) accepts the IP address for the lease term.

2. NAT

Stands for "Network Address Translation." NAT translates the IP addresses of computers in a local network to a single IP address. This address is often used by the router that connects the computers to the Internet. The router can be connected to a DSL modem, cable modem, T1 line, or even a dial-up modem. When other computers on the Internet attempt to access computers within the local network, they only see the IP address of the router. This adds an extra level of security, since the router can be configured as a firewall, only allowing authorized systems to access the computers within the network.

Once a system from outside the network has been allowed to access a computer within the network, the IP address is then translated from the routers address to the computers unique address. The address is found in a "NAT table" that defines the internal IP addresses of computers on the network. The NAT table also defines the global address seen by computers outside the network. Even though each computer within the local network has a specific IP address, external systems can only see one IP address when connecting to any of the computers within the network.

To simplify, network address translation makes computers outside the local area network (LAN) see only one IP address, while computers within the network can see each systems unique address. While this aids in network security, it also limits the number of IP addresses needed by companies and organizations. Using NAT, even large companies with thousands of computers can use a single IP address for connecting to the Internet. Now thats efficient.

 NAT has many forms and can work in several ways:

• Static NAT - Mapping an unregistered IP address to a registered IP address on a one-to-one basis. Particularly useful when a device needs to be accessible from outside the network.
• Dynamic NAT - Maps an unregistered IP address to a registered IP address from a group of registered IP addresses.

• Overloading - A form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address by using different ports. This is known also as PAT (Port Address Translation), single address NAT or port-level multiplexed NAT.

• Overlapping - When the IP addresses used on your internal network are registered IP addresses in use on another network, the router must maintain a lookup table of these addresses so that it can intercept them and replace them with registered unique IP addresses. It is important to note that the NAT router must translate the "internal" addresses to registered unique addresses as well as translate the "external" registered addresses to addresses that are unique to the private network. 

This can be done either through static NAT or by using DNS and The internal network is usually aLAN (Local Area Network), commonly referred to as the stub domain. A stub domain is a LAN that uses IP addresses internally. Most of the network traffic in a stub domain is local, so it doesnt travel outside the internal network. A stub domain can include both registered and unregistered IP addresses. Of course, any computers that use unregistered IP addresses must use Network Address Translation to communicate with the rest of the world.





3. Subnetting

A subnet is a logical grouping of connected network devices. Nodes on a subnet tend to be located in close physical proximity to each other on a LAN.

Network designers employ subnets as a way to partition networks into logical segments for greater ease of administration. When subnets are properly implemented, both the performance and security of networks can be improved.

In Internet Protocol (IP) networking, devices on a subnet share contiguous ranges of IP address numbers.

A mask (known as the subnet mask or network mask) defines the boundaries of an IP subnet. The correspondence between subnet masks and IP address ranges follows defined mathematical formulas. IT professionals use subnet calculators to map between masks and addresses.

4. IPv4

The Internet Protocol version 4 was designed to be allocated to approx. imately 4.3 billion addresses. At the beginning of Internet this was considered a much wider address space for which there was nothing to worry about.

The sudden growth in internet users and its wide spread use has exponentially increased the number of devices which needs real and unique IP to be able to communicate. Gradually, an IPS is required by almost every digital equipment which were made to ease human life, such as Mobile Phones, Cars and other electronic devices. The number of devices (other than computers/routers) expanded the demand for extra IP addresses, which were not considered earlier.

Allocation of IPv4 is globally managed by Internet Assigned Numbers Authority (IANA) under coordination with the Internet Corporation for Assigned Names and Numbers (ICANN). IANA works closely with Regional Internet Registries, which in turns are responsible for efficiently distributing IP addresses in their territories. There are five such RIRS. According to IANA reports, all the IPv4 address blocks have been allocated. To cope up with the situation, the following practices were being done:

• Private IPs: Few blocks of IPs were declared for private use within a LAN so that the requirement for public IP addresses can be reduced.
• NAT: Network address translation is a mechanism by which multiple PCs/hosts with private IP addresses are enabled to access using one or few public IP addresses.
• Unused Public IPs were reclaimed by RIRs.

5. IPv6

IETF (Internet Engineering Task Force) has redesigned IP addresses to mitigate the drawbacks of IPv4. The new IP address is version 6 which is 128-bit address, by which every single inch of the earth can be given millions of IP addresses.

Today majority of devices running on Internet are using IPv4 and it is not possible to shift them to IPv6 in the coming days. There are mechanisms provided by IPv6, by which IPv4 and IPv6 can co-exist unless the Internet entirely shifts to IPv6:

• Dual IP Stack
• Tunneling (6to4 and 4to6)
• NAT Protocol Translation

6. Public Vs Private IP

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