Network Services
Objectives:
● 1.2 - Compare and contrast networking appliances, applications, and functions
● 3.4 - Given a scenario, implement IPv4 and IPv6 network services
● Introduction
○ Network Service
■ Function provided by the network infrastructure to support various types
of communications and processes
● Dynamic Host Configuration Protocol (DHCP)
○ Dynamic Host Configuration Protocol (DHCP)
■ Automates the process of assigning IP addresses to devices on a network
■ Helps prevent configuration errors and IP conflicts by dynamically
assigning IPs
○ Scope
■ A range of valid IP addresses available for assignment on a subnet
■ Devices automatically receive an IP address from the scope when joining
the network
■ Administrators can exclude certain IPs or use reservations for specific
devices
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○ DHCP Lease
■ DHCP leases IPs for a specific period (default is 24 hours)
■ Longer lease times are common in corporate networks for stability and
security
○ DHCP Reservation
■ Excludes some IP addresses from being handed out to devices unless a
certain condition is met
○ DHCP Process: DORA
■ Discover
● Device requests an IP address from the DHCP server
■ Offer
● DHCP server offers an IP address to the device
■ Request
● Device requests the offered IP address
■ Acknowledge
● DHCP server acknowledges the request and assigns the IP
○ DHCP Configuration
■ DHCP assigns four key pieces of information to devices
● IP address
● Subnet mask
● Default gateway IP
● DNS server IP
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■ DHCP set up:
● Dynamic assignment
○ DHCP servers do the configuration
● Static assignment
○ Requires manual configuration of all four pieces of
information
■ An alternate configuration is used if DHCP fails
● APIPA – default
● Static IP
■ DHCP Scope Options
● DHCP servers can be configured with scope options
○ Subnet mask
○ Default gateway
○ DNS server
○ Lease time
■ DHCP Relay
● Any host that forwards DHCP packets between clients and servers
● Useful when clients and servers are not on the same subnet
● Avoids installing DHCP servers on every subnet
■ UDP and IP Helper Address
● DHCP operates using UDP
○ A fire-and-forget method of sending data
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● IP Helper Address
○ Used to forward UDP broadcasts, ensuring DHCP requests
reach the server
● If the DHCP client and server are on separate network segments,
the client's network segment router must be configured with an IP
helper address
○ This configuration enables the router to properly forward
DHCP requests to the DHCP server
● Understanding DHCP: Demonstration
● Stateless Address Autoconfiguration (SLAAC)
○ Stateless Address Auto Configuration (SLAAC)
■ An integral component of the IPv6 network protocol, simplifying the
network configuration process by allowing devices to autonomously
assign their IP addresses
■ Created to reduce administrative overhead and foster self-sufficiency in
network management
○ SLAAC Advantages
■ Enhances network efficiency
■ Eliminates potential for IP conflicts
■ Streamlines integration of new devices into networks
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○ Operation
■ Device Initiation
● Device generates a temporary link-local address
■ Router Solicitation
● Device sends a message to identify local routers
■ Router Advertisement
● Routers respond with network prefix information
■ Address Configuration
● Device combines prefix with unique identifier to create IP address
■ Neighbor Solicitation (Final check)
● Device checks for address conflicts before settling on IP address
○ Real-World Analogy
■ Similar to smart devices in a home autonomously finding their place on a
network without manual assignment
● Domain Name System (DNS)
○ Domain Name System (DNS)
■ A protocol used to convert human-readable domain names (like
example.com) into IP addresses (like 192.0.2.1) that computers use to
identify each other on the network
■ Helps users find websites using easy-to-remember domain names instead
of numeric IP addresses
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○ Functionality
■ When a user's computer wants to access a website, it contacts a DNS
server to resolve the domain name to an IP address
■ DNS servers store mappings of domain names to IP addresses in records
○ Fully Qualified Domain Name (FQDN)
■ A domain name that is under a top-level provider
● .com – most common
● .mil
● .edu
● .org
● .net
○ DNS is structured in a five-level hierarchy
■ Root Level
● Highest level
● Answers requests in the root zone
■ Top-Level Domain (TLD)
● Organizational hierarchies – .com, .net, .org, etc.
● Geographical hierarchies – .uk (United Kingdom), .it (Italy), .fr
(France), etc.
■ Second-Level Domain
● Tied directly below the top level domain (e.g., diontraining in
diontraining.com)
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■ Subdomain
● Server underneath second-level domain (e.g., www in
www.diontraining.com)
■ Host
● Lowest and most detailed level
● Refers to a specific machine within a domain
○ URL (Uniform Resource Locator)
■ Specifies how to access a resource (e.g., a website) on the internet using
a domain name and a protocol (e.g., https://www.diontraining.com)
■ Examples
● Secure – https://
● Insecure – http://
● FTP – ftp://
○ Host File
■ A simple text file that serves as the first point of contact when the device
wants to seek out communication with other devices over the network
■ Can be used to manually map domain names to IP addresses, bypassing
the need for DNS
■ Takes priority over DNS and has a closer proximity
■ Useful for testing and can be a security risk if not managed properly
■ Can be used as a temporary workaround for DNS issues
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○ Security Implications
■ DNS can be used maliciously to redirect users to fake websites for
phishing attacks
■ Manipulating the hosts file can also be a security risk if not done carefully
● DNS Record Types
○ DNS Record Types
■ A Records (Address Records)
● Link host names to IPv4 addresses
■ AAAA Records (IPv6 Address Records)
● Link host names to IPv6 addresses
■ CNAME Records (Canonical Name Records)
● Used to point a domain to another domain name or subdomain
● Example
○ Setting multiple domains to resolve to the same site
● Cannot be used to point to an IP address
■ MX Records (Mail Exchange Records)
● Direct emails to a mail server using SMTP over port 25
● Point to another domain, not an IP address
● Include priority values to determine preferred servers
○ The lower the number, the higher the priority
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○ To load balance email across multiple servers, set their
priorities to be the same value
■ SOA Records (Start of Authority Records)
● Store important domain information like update times and
administrators
● Critical for DNS server zone transfers
○ Zone Transfer
■ Process of sending DNS records data from the
primary nameserver to a secondary nameserver
■ Uses TCP
■ PTR Records (Pointer Records)
● Correlate an IP address with a domain name
● Opposite of A Records
● Reverse DNS lookup
○ Determines what the domain name is for a given IP
address
○ Query is made against PTR records
○ Forward Lookup
■ Uses DNS to find the IP address for a given domain
name
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● Helpful in proving that a domain is not associated with spam,
troubleshooting email deliverability issues, or creating better
logging by converting IP addresses back into domain names
● Always stored under .arpa top-level domain
○ Versed as “.in-addr.arpa”
○ Advanced Research Projects Agency Network (ARPAnet)
■ First top-level domain that was defined for what
would become the Internet
■ TXT Records (Text Records)
● Add text into the DNS
● Store human-readable or machine-readable data
● Used for domain ownership verification and email spam
prevention
■ NS Records (Nameserver Records)
● Indicate the authoritative DNS name server for a domain
● Necessary for DNS hierarchy and record management
● Nameserver
○ Type of DNS server that stores the DNS records for a given
domain
○ There are often more than one nameserver for domain
■ Primary
■ Secondary
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○ Internal and External DNS
■ Internal DNS
● For private networks, cloud instances, etc.
■ External DNS
● For publicly accessible domain names
○ TTL (Time to Live)
■ Specifies how long a DNS resolver should cache a query before requesting
a new one
● DNS Resolver/DNS Cache
○ Makes local copy of every DNS entry it resolves as the user
connects to websites
○ Helps in speeding up DNS lookups and managing server
load
○ Recursive Lookup
■ DNS resolver keeps querying DNS servers until it finds the IP for a domain
■ Used for finding IPs for domain names
○ Iterative Lookup
■ DNS server tells the resolver to query the next DNS server until it finds
the IP
■ Resolver is responsible for hunting down the IP
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● Securing DNS
○ Domain Name System (DNS)
■ Redirects network devices from domain names to associated IP addresses
■ Critical for network security
● If compromised, can redirect requests to malicious servers
○ DNS Security Extensions (DNSSEC)
■ Provides a digital seal for DNS data to ensure it hasn't been tampered
with
■ Uses cryptographic signatures verified against a chain of trust
■ Prevents successful exploitation even if DNS records are falsified
■ Does not encrypt DNS data
○ DNS over HTTPS (DoH)
■ Encrypts DNS queries sent through HTTPS
■ Blends DNS queries with HTTPS traffic for increased privacy and security
○ DNS over TLS (DoT)
■ Encapsulates DNS traffic in a Transport Layer Security (TLS) tunnel
■ Encrypts DNS data for privacy, preventing eavesdropping on queries
○ Privacy and Integrity
■ DNS Snooping
● An attacker monitors DNS queries to infer what websites a user is
visiting
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● Prevented by both DoH and DoT, ensuring online activities remain
private
○ Trust and Integrity
■ Implementing DNSSEC, DoH, and DoT safeguards trust in the digital
ecosystem
○ Implementation Considerations
■ Collaborative Effort
● Requires involvement of website owners, ISPs, and organizations
■ Balancing Act
● Implementing DoH may shift DNS resolution control to third-party
providers, requiring careful consideration
○ Overall Objective
■ While protecting data, also aim to preserve trust and reliability in
networks and the internet
● Understanding DNS: Demonstration
● Network Time Protocol (NTP)
○ Network Time Protocol (NTP)
■ Used for synchronization of clocks between different computer systems
■ Sends data using UDP packets on port 123
■ NTP version 4 (2010) – most current version
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■ Synchronizes clocks to within a few milliseconds of Coordinated Universal
Time (UTC)
■ Components
● Stratum
○ Hierarchical system of time sources
○ Stratum 0
■ Most precise timekeeping devices like atomic
clocks
■ Reference clock
○ Stratum 1
■ Where first NTP servers in the hierarchy are going
to start at
■ Computers synchronized to within a few
microseconds of the attached stratum 0 devices
■ Considered primary time servers
○ Stratum 2
■ Connected and synchronized to stratum 1 servers
■ Often configured to query multiple stratum 1
servers
○ Stratum 3
■ Synchronized upward back to stratum 1 and 2
servers
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○ The hierarchy continues with each stratum adding more
delay and becoming further from stratum 0
○ Stratum 15 – maximum limit
○ Stratum 16
■ Indicates unsynchronized devices
● Clients
● Servers
■ In enterprise networks, a time server is connected, and client software is
installed on workstations to interface with the server
■ NTP service on domain controllers acts as the time source for
workstations
○ Precision Time Protocol (PTP)
■ Used to synchronize clocks throughout the computer network
■ Achieves clock accuracy in the sub-microsecond range
■ Ideal for networks requiring precise timekeeping
● Financial trading
● Industrial automation systems
■ Uses a primary-secondary architecture for clock synchronization
● Primary clocks send precise time messages
● Secondary clocks adjust to align with the primary clock
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○ Network Time Security Protocol (NTS)
■ An extension of NTP developed to provide cryptographic security for time
synchronization
■ Uses TLS and AEAD to ensure secure time synchronization
■ Authenticates the time source and the integrity of received time to
prevent malicious tampering
● Quality of Service (QoS)
○ Quality of Service (QoS)
■ Enables strategic optimization of network performance based on different
types of traffic
■ Purpose
● Ensure proper delivery of voice, data, and video content over
converged networks
■ Benefits
● Optimize network performance
● Efficiently utilize bandwidth
● Ensure service availability
● Save costs
○ QoS Components
■ Categorization
● Identify and categorize traffic types (e.g., web, voice, video, email)
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■ Prioritization
● Assign priority levels to different types of traffic
■ Bandwidth Management
● Determine and allocate required bandwidth for each traffic type
■ Traffic Dropping
● Identify and drop lower-priority traffic during congestion to
maintain quality for higher-priority traffic
○ QoS Categories
■ Delay
● Time taken for a packet to travel from source to destination
○ Measured in milliseconds
● Critical for real-time traffic like voice and video
■ Jitter
● Uneven arrival of packets
● Particularly detrimental to voice over IP (VoIP) traffic
■ Drops
● Occur during network congestion
○ Leads to packet loss
● More critical for UDP-based traffic like VoIP
○ Effective Bandwidth
■ The lowest bandwidth among the links in a network path, which
determines the actual data rate
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■ Limits the overall throughput of the network, impacting user experience
and application performance
● QoS Categorization
○ Purpose of QoS
■ Categorize traffic into buckets
■ Apply policies based on traffic categories
■ Prioritize traffic based on categories
○ Traffic Prioritization
■ High priority for real-time traffic like VoIP to avoid delays and ensure
quality
■ Low priority for non-mission-critical data like web browsing or email
■ Documentation and Communication
● Document and share QoS policies with users
● Help users understand the policies to avoid confusion and
reporting of issues
○ Mechanisms of Traffic Categorization
■ Best Effort
● No QoS
● First in, first out
● No reordering or shaping
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■ Integrated Services (IntServ)
● Hard QoS
● Strict bandwidth reservations for different traffic types
■ Differentiated Services (DiffServ)
● Soft QoS
● Traffic marked for different priorities
● Allows for flexible allocation
○ Comparison of QoS Approaches
■ Best Effort
● Least efficient, no strict policies.
■ DiffServ (Soft QoS)
● Better than Best Effort but less efficient than IntServ
■ IntServ (Hard QoS)
● Strict policies, highest level of service for critical traffic
○ Implementation of QoS
■ Classification and marking
■ Congestion management and avoidance
■ Policing and shaping
■ Link efficiency
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● QoS Mechanisms
○ Traffic Categorization
■ Classification
● Determines traffic categories based on type (e.g., email protocols)
● Analyzes packet headers, packet type, and ports
● Helps prioritize services without altering packet bits
■ Marking
● Alters bits within frames, cells, or packets
● Indicate how to handle traffic
● Uses Type of Service (ToS) header
○ IP precedence
○ DSCP
■ Congestion Management
● Queuing
○ Buffers extra traffic when devices receive data faster than
they can transmit
○ Empties the packets in specified sequence and amount
using one of three mechanisms:
■ Weighted Fair Queuing
■ Low-latency Queuing
■ Weighted Round-robin
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■ Congestion Avoidance
● RED (Random Early Detection)
○ Prevents buffer overflow by dropping packets based on
priority
○ Discards lower priority packets first to avoid congestion
■ Policing and Shaping
● Policing
○ Discards packets that exceed configured rate limits
○ Results in retransmissions, creating more bandwidth
○ Good for very high speed interfaces
● Shaping
○ Delays traffic from exceeding rate limits by buffering
○ Holds packets in the buffer and releases them when space
is available
■ Link Efficiency
● Compression
○ Reduces packet size, conserving bandwidth
○ Utilizes protocols like CRTP (Compressed RTP) for VoIP to
reduce header size significantly
● LFI (Link Fragmentation and Interleaving)
○ Fragments big packets and interleaves smaller packets to
utilize slower links efficientl