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Subnet Masks and DCHP - Gateways

• - Computers in different types of networks - ones that use different network protocols, or architecture - frequently need to communicate with each other.
• - Gateways, which combine hardware and software, are used to allow communication between dissimilar networks.
• - Gateways convert data into a format that the receiving network can process. In order to operate properly, they must be able to work on multiple levels of the Open Systems Interconnection, or OSI, model.
• - Because gateways perform complex tasks such as translating encoded data, they're sometimes slow and and can cause network congestion. However, they're essential in some situations.

5 Main Types of Gateways

• - Emails: E-mail gateways translate and exchange mail between two different types of e-mail systems. For example, an e-mail gateway can send messages from a server that uses Postfix mail server to one that uses Microsoft Exchange Server.
• - Internet: Internet gateways control communication between networks and the internet. For example, an Internet gateway can be used to control Internet resources that users on a LAN are able to access.
• - LAN: LAN gateways enable LAN components operating on different protocols to communicate with each other. For example, remote users dialling into a network can gain access to a network that uses different protocols than the machine that they're connecting from. Routers typically act as gateways in LANS.
• - Voice and Data: Voice and data gateways connect the sections of a network that handle voice traffic and data traffic. Voice applications and data applications function very differently. For voice signals to be integrated into data networks, and vice versa, they have to be translated and compressed. This requires specific protocols and processes, which are performed by the voice and data gateway.
• - Firewall: Firewalls are gateways that control traffic entering and leaving networks. Firewalls can be software or hardware-based, or a combination. They allow or deny access to packets using rules that have been established by a network administrator or by an algorithm designed to detect and block malicious traffic.

Subnet

• - Network administrators often break large networks down into smaller virtual segments called subnets. In addition to providing links between dissimilar networks, gateways may link subnets wishing a single network.
• - Every network ID has an associated subnet mask. This is a number compromised of four octets, which are used to identify how a network is divided.
• - Using a subnet mask, you can determine which parts of an IP address identify network information and which parts identify host information.
• - In a default glassful subnet masks, network information is always represented by setting all the bits in an octet to 1, and host information is represented by setting all the bits in an octet to 0. Therefore, subnet octets are a combination of the decimal number 255, 11111111, and 0, or 00000000.

Subnet Mask

• - Three classes of networks have a default subnet mask when they aren't subdivided:
• - CLASS A (255.0.0.0): Class A networks have a default subnet mask of 255.0.0.0. The binary equivalent of the Class A network's default subnet mask is an octet of one and three octets of zeroes.
• - CLASS B (255.255.0.0): Class B networks have a default subnet mask of 255.255.0.0. The binary equivalent of the Class B network's default subnet mask is two octets of ones and two octets of zeros.
• - CLASS C (255.255.255.0): Class C networks have a defaults subnet mask of 255.255.255.0. The binary equivalent of the Class C networks'd default subnet mask is three octets of ones and one octet of zeros.

Slide 5

• When a network is subnetted, bits within the default subnet mask that are usually used for host address information are changed to represent network information. The changes to these bits create new unique identifiers for each subnet. Changing a single bit divides a network into smaller segments

Slide 6

• - Globally unique IP addresses are controlled by government-funded organisation. These organisations are in charge of allocating IP addresses to network providers and Internet service providers, or ISPs.
• - Government
• ... Network Providers
• ... Internet Service Providers
• ISPs then allocate groups of addresses to companies and individuals that require them. The IP addresses that a company owns form a pool of addresses. From this pool, each device that's part of the company's LAN may be assigned an individual address.

Methods to Configure IP Addresses

• There are three main methods that you can use to configure IP addresses on hosts:
• - Static: Static IP addresses are configured manually on each device. Although they can be assigned to any type of device, it's recommended that you assign them only to network devices such as routers, servers and printers, not to clients. Each device that's assigned a static IP address needs to have at least and a subnet masks. For subnet access to WANS such as the Internet, you need to configure a device's IP address, a subnet mask, a default gateway, and the Domain Name System, or DNS, server address.
• - Dynamic: Dynamic IP addresses are assigned to devices automatically when they start up, usually using a Dynamic Host Configuration Protocol, or DHCP, server. Whereas static IP addresses are used to configure network devices such as routers, servers, and printers, dynamic IP addresses are normally assigned to clients in a network. Using dynamic addressing for these devices greatly enhances security and increases network manageability. because users aren't involved in configuring addresses, and support staff don't have to go from desk-to-desk configuring addressed.
• - Self-Assigned: Self-assigned IP addresses are used when a client is unable to contact a DCHP server to retrieve a dynamic IP address. On Microsoft Windows 98 and later clients, self-assigned addresses are provided via a service called Automatic Private IP addressing or APIPA. APIPA automatically assigns a client a unique IP address, for use until a DHCP address is available. However, self-assigned IP addresses can't enable devices to access other subnets, networks or the Internet.

Fully Qualified Domain Names

• - Instead of IP addresses, Fully Qualified Domain Names, or FQDNs, are used in Internet or TCP/IP-based applications, such as web browsers.
• - When you attempt to connect to another machine using it FQDN, DNS consults a large distributed database and resolves the FQDN into an IP address for that machine.

Local Resolver Cache

• - When using a DNS, a client first attempts name resolution using a local resolver cache, which stores recently resolved host names. The contents of the cache can be accessed quickly, and using the cache prevents additional network traffic from being created.

BOOTP & DHCP

• _ Dynamic Host Configuration Protocol, know as DHCP, was developed to replace the Bootstrap Protocol, or BOOTP.
• - BOOTP is a protocol that clients in a network can use to obtain an IP address from a server. However, using this protocol, network administrators needed to maintain tables of IP addresses and MAC addresses on BOOTP servers.
• - DHCP enable each network device to borrow or lease an IP address for a specified period of time. This period depends on the configuration of the client device and of the DHCP server. When a lease expires, a client has to renegotiate the lease before acquiring an IP address.
• - A lease may also be terminated early by a network administrator or by the client.

The Introduction of DHCP enabled:

• The Introduction of DHCP enabled:
• - A reduction in tome and management requirements: DHCP helps to reduce administrative overhead. Using DHCP, network administrators no longer needed to manage IP addresses centrally on a server or TCP/IP configurations individually on any network devices.
• - A reduction in errors: Using DHCP, there's a reduced possibility that an invalid IP address will be assigned to a device or that two devices will be assigned the same IP address.
• - Greater user mobility: Using DHCP, you can move any network device that has an IP address to anywhere else within a network without having to change its TCP/IP configuration. This is because a device can simply request its IP address from the central server when it boots up on the new location.
• - Transparent Mobile Addressing for Mobile Users: Using DHCP, a mobile user can simply plug a device into a network and the device will automatically request its IP address from the central server. This is especially useful in an office environment. It enables visitors to attach laptops to a network and receive an IP address without having to change their laptop configurations.

Configuring DHCP

• - Before you can use DHCP on a network, you have to configure a DHCP server with a range - or pool - of IP addresses that it can be assign to network devices.
• - You also have to configure a lease duration. This can range from minutes to an infinite duration.
• - As soon as the DHCP server is configured and running properly, the server and network clients can begin negotiating leases. Lease negotiation occurs as soon as a client device connects to the network.

DHCP - Negotiating a lease - Steps

• The client and the server negotiate a lease using a series of steps:
• 1. DHCP disover : As soon as the client device operating system starts up, its network interface card, or NIC, detects a network connection. It then uses the the User Datagram Protocol, or UDP, to broadcast a DHCP discover packet in an attempt to locate a DHCP server.
• 2. DHCP offer: All the DHCP servers within the same subnet receive the client device's broadcast. Each responds by identifying an available IP address from its pool and sending a message containing this address, a subnet mask, its own IP address and the lease duration to client. At the same time, each DCHP server holds back the IP address it has offered to the client from any other potential requests.
• 3. DHCP request: The client accepts the first address that it receives and sends a broadcast message of confirmation to the DHCP server. Within this message, the client may also request any additional information. The other DHCP servers in the subnet also receive this message, and return their rejected IP addresses to their address pools.
• 4. DHCP acknowledgement: The selected DHCP server responds to the client's request message with an acknowledgement message that contains any additional information the client has requested.

Slide 14

• The IP address that the client has been assigned remains configured even if the client shuts down or moves position within the network.
• When the client wishes to join a new network, the IP address should be replaces with a new IP address that's specific to that network.
• APIPA, which automatically assigns client devices IP addresses if no DHCP server is reachable, is available on Microsoft Windows 98 and later operating systemss.