CompTIA® Network+ Exam Notes : Network Services

1. Networking Concepts

1.8 Network Services

DHCP Service: DHCP scopes enables ranges of IP addresses, to be defined on a system running a DHCP server application. When a network device is assigned an IP address from an appropriate DHCP scope, that assignment is not permanent. Rather, it is a temporary assignment referred to as a lease . A specific MAC address is mapped to a specific IP address, which will not be assigned to any other network device. This static addressing approach is referred to as a DHCP reservation .

The Internet Protocol Helper (IP Helper) API enables the retrieval and modification of network configuration settings for the local computer.

DHCP scopes enables ranges of IP addresses, to be defined on a system running a DHCP server application.

DHCP relay forwards DHCP requests between the server and the clients.

When a network device is assigned an IP address from an appropriate DHCP scope, that assignment is not permanent. Rather, it is a temporary assignment referred to as a lease .

DHCP Relay: As DHCP messages are broadcast, these broadcast messages are not forwarded out of subnet by router to other subnets (Assuming that you have more than one subnet). It means you need to have a DHCP server for each of your subnet, which is not practical. The DHCP relay agent receives DHCP Discover and Request messages broadcasted by the PC, and unicasts them directly to the DHCP server. With each of your subnets configured with a relay agent, one DHCP server can cater to all the clients in different subnets.

Note: There is no difference between the two DHCP Relay and IP Helper Address. DHCP Relay essentially is the ability of a host to forward DHCP packets between clients and servers, when they reside on different subnet. ip helper-address is the cisco syntax for enabling DHCP relaying.

When a network device is assigned an IP address from an appropriate DHCP scope, that assignment is not permanent. Rather, it is a temporary assignment referred to as a lease .

DNS Services: The core administrative unit in DNS is called a "zone". A zone is a physical file composed of resource records that define a group of domains. A domain is a node in the DNS name-space and all sub-domains below it. DNS is used for host name to IP address resolution.

Any web server can be made to be present on the public Internet by using an external DNS name and a public IP address. Of course, you need to ensure physical connectivity to the Internet, and a broadband router with ISP connection is a good solution for this. Web server listens to port 80, therefore, you need to enable port 80 (incoming) on the router. HOSTS file is used like DNS for resolving Domain names, whereas LMHOSTS file is used like WINS for resolving NetBIOS names. Note that, if you are using HOSTS file for resolving Domain names, you need to load the HOSTS file on all computers on the network that need to access the domain names. This is in contrast with DNS, which is located on a central server.

The IP address and domain name go in one line for ex:

Also note that the HOSTS file is an ASCII file and can be edited using Notepad or any other text editor. HOSTS file resolves the domain name to corresponding IP address. The DNS server needs to be specified for connecting using FQDN (Fully Qualified Domain Name).

The most common type of DNS record is a Host record (also called an A record). In the Internet, a Host record is used to associate a domain name (FQDN - Fully Qualified Domain Name) with an IP address.

An MX record stores the IP address of your SMTP server, so e-mail clients can determine where a message should be sent. They perform a DNS query against a domain's MX record to get the IP address of the organization's SMTP server.

An Alias record's job is to associate an alternate name with a computer for which there is already a Host record. For example, suppose that the host record for relevant looked like this: Relevant Host (A)

Alias record is like a redirect. For example, you have a site However, someone enters, You want him or her to be directed to the Since there is no server on the network named "www," set up an Alias record that associates www with The Alias record looks something like this: www Alias (CNAME)

WINS resolves NetBIOS names to IP addresses.

DNS Record Types:

A - specifies IP addresses corresponding to your domain and its subdomains

MX - specifies where the emails for your domain should be delivered

CNAME - specifies redirects from your domain's subdomains to other domains/subdomains

TXT - these records are used to store text-based information related to your domain. One of their most common uses is for SPF data. SPF, or Sender Policy Framework

SRV - Each SRV record has the following form:

where each of the attributes should have a specific value:

- service - the name of the service in question.

- proto - the protocol of the service (usually TCP or UDP).

- name - the domain name that the record should be valid for (Note: it should end with a dot).

- TTL - time to live

- priority - lower values mean more preferred.

- weight - used for records with the same priority (higher value means more preferred).

- port - the number of the port.

- target - the hostname of the machine where the service is running (Note: it should end with a dot).

TTL, Priority, Weight, Port and Target are straight forward and easy to fill out. The values will be given by the company that will generate the SRV Record for you.

Here is an example of an SRV record: 3600 IN SRV 10 60 5060

IPAM: IP Address Management (IPAM) is software that can track, plan, and manage IP addressing on networks. It is widely available from vendors and third-parties. Microsoft's implementation automatically discovers IP address infrastructure and DNS servers on your network, enabling you to manage them from a central interface.

NTP: The Network Time Protocol (NTP) is one of the oldest protocols and is used to synchronize the time on packet switched networks. Time synchronization is crucial to systems and processes that update data. A program may ignore your data update if the update is older than its latest saved data.


WINS is used for NetBIOS name resolution. DHCP is used for assigning IP addresses to client computers dynamically. SNMP is used for remote monitoring of a TCP/IP network.

1.9 Network Architecture

Three Tiered Architecture

To improve system performance, as well as to improve security, it is possible to implement a tiered systems model. This is often referred to as an n-tiered model because the n- can be one of several different numbers. If we were looking at database, for example, with a one-tier model, or single tier environment, the database and the application exist on a single system. This is common on desktop systems running a standalone database. Early UNIX implementations also worked in this manner; each user would sign on to a terminal and run a dedicated application that accessed the data. With two-tier architecture, the client workstation or system runs an application that communicates with the database that is running on a different server. This common implementation works well for many applications. With three-tiered architecture, security is enhanced. In this model, the end user is effectively isolated from the database by the introduction of a middle-tier server. This server accepts requests from clients, evaluates them, and then sends them on to the database server for processing. The database server sends the data back to the middle tier server, which then sends the data to the client system.

Core Layer:The core layer is the backbone: the place where switching and routing meet (switching ends, routing begins). It provides high-speed, highly redundant forwarding services to move packets between distribution-layer devices in different regions of the network.

Distribution/Aggregation Layer:The distribution layer, or aggregation layer (sometimes called the workgroup layer), is the layer in which management takes place. This is the place where QoS policies are managed, filtering is done, and routing takes place.

Access/Edge Layer: Switches that allow end users and servers to connect to the enterprise are called access switches or edge switches, and the layer where they operate in the three tiered model is known as the access layer, or edge layer. Devices at this layer may or may not provide Layer 3 switching services; the focus is on minimizing the cost of each provisioned Ethernet port (known as "cost-perport") and providing high port density.

Software-Defined Networking (SDN) is an architecture that centralizes the brains of all networking devices into a single component. All the networking devices are operated through this central component. The management of packet transmission is performed by the same central entity and the networking devices behave as ‘forwarding devices’, which receive and forward packets. In technical terms, SDN dissects the control planes from networking devices and centralizes them to a single component, and the networking devices are left out with only data planes that are responsible for forwarding packets.

SDN Architecture: SDN architecture consists of three major layers:

Application Layer: It is responsible for developing and managing networking applications. Application layer deals with end-user business applications that utilizes the SDN services. Business application such as energy efficient networking, security monitoring, network virtualization etc.

Control Layer: The control layer is also referred as control plane that comprises a set of software-enabled SDN controllers. This layer allows the network administrator to apply custom policies to the physical layer devices. It is responsible for routing packets to their respective destination while applying different rules and policies.

Infrastructure Layer: This layer consists of forwarding devices like the physical switch, router, etc. Software switches which can be accessible via open interfaces, also part of this layer. This layer is considered as forwarding layer since it allows packet switching and forwarding.

Spine and Leaf Architecture The spine-leaf architecture consists of only two layers of switches spine and leaf switches. The spine layer consists of switches that perform routing and work as the core of the network. The leaf layer involves access switches that connect to servers, storage devices, and other end-users. This structure helps data center networks reduce hop count and reduce network latency.

Spine Layer – serves as the backbone of the network similar to the core layer in our three-tier design. It is where we can find the spine switches which can be a Layer 3 switch. Each Layer 3 port is connected to the underlying Layer 2 leaf switch.

Leaf Layer – connects to end devices similar to the access layer in our three-tier design. It is where the leaf switches which connect to all spine switches reside. In our example above, we have servers that connect to leaf switches. In a data center environment, it can be any kind of server, like web server, application server, database server, or storage server.

Benefits of Spine and Leaf Architecture

Below are the advantages and benefits of implementing spine-leaf architecture in the network:

Improved Redundancy – As opposed to our traditional three-tier architecture where access layer switches connect to only two uplink distribution switches, every leaf switch connects to every spine switch. And instead of Spanning Tree Protocol (STP), we implement Transparent Interconnection of Lots of Links (TRILL) and Shortest Path Bridging (SPB), which allows traffic flows across all available links, offering improved redundancy, but like STP, still prevent loops.

Increased Bandwidth – By implementing TRILL and SPB, we have the ability to use multiple active links instead of one and it increases bandwidth. With STP, only one link is active and the other links are blocked.

Improved Scalability – In the event of over subscription, we can add a spine switch and connect it to every leaf switch. If the port density is a concern, we can add a leaf switch and connect it to every spine switch.

Lower Costs – Fixed-configuration switches unlike modular switches, have a fixed number of ports and are usually not expandable. Many spine-leaf networks use fixed-configuration switches.

Low Latency and Congestion Avoidance – With having only a maximum of two hops between any source and destination nodes, we make a more direct traffic path, which improves performance and reduces bottleneck. The only exception is when the destination is on the same leaf switch.

Energy Efficient – Fixed-configuration switches require slightly lower power consumption.

Limitations of Spine and Leaf Architecture

As we have advantages and benefits, we also have limitations in implementing spine-leaf architecture in our network:

Amount of Cables – We need to run more copper or fiber cables since each leaf must be connected to every spine device.

Limited Hosts – The number of hosts that we can support can be limited. Spine port counts can restrict the number of leaf switch connections.

As technology advances, more data center traffic is moving from server to server instead of moving in and out of the data center. As network engineers or future network engineers, we should learn to plan, build and manage spine-leaf architecture.

Traffic Flows: Traffic flows within a datacenter typically occur within the framework of one of two models: East-West or North-South. The names may not be the most intuitive, but the East-West traffic model means that data is flowing among devices within a specific datacenter while North-South means that data is flowing into the datacenter (from a system physically outside the datacenter) or out of it (to a system physically outside the datacenter).

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