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CompTIA Network+ (N10-009) · chapter 16 of 29

Week 16: Addresses on tap

In this chapter Objectives 3.4

  1. DHCP
  2. IPv6 addressing: SLAAC
  3. Practice questions
  4. Chapter recap

Objective 3.4

DHCP

Scope, lease time, exclusions

A scope is the range of addresses a DHCP server hands out for one subnet, plus its settings. The lease time is how long a client may keep an address; clients try to renew at 50% of the lease. Exclusions are addresses inside the scope the server must never hand out, such as ones you assigned statically to printers or the gateway.

Scope 10.20.40.0/24  CAMERAS-S2
Range       10.20.40.10 - 10.20.40.250
Exclusions  10.20.40.10 - 10.20.40.19
Lease       8 days

Exam tip Short leases suit busy guest Wi-Fi (addresses return fast). Long leases suit stable wired devices.

Reservations

A reservation ties one address to one client's MAC address, so that device always gets the same IP from DHCP. You get a predictable address but still manage it centrally (options update automatically).

Exam tip Reservation = DHCP always gives this MAC this IP. Exclusion = DHCP never gives this IP to anyone.

DHCP options

Options are the extra settings handed out with the address. Common ones:

  • Option 3: default gateway (router)
  • Option 6: DNS servers
  • Option 15: DNS domain name
  • Option 51: lease time
  • Option 66/150: TFTP server for IP phones (150 is Cisco)

Relay / IP helper

DHCP Discover is a broadcast, and routers do not forward broadcasts. A DHCP relay (Cisco: ip helper-address) on the router interface catches the broadcast and forwards it as unicast to the DHCP server in another subnet.

interface Vlan40
 ip address 10.20.40.1 255.255.255.0
 ip helper-address 10.1.8.5
A client in VLAN 40 broadcasts a DHCP Discover. Routers do not forward broadcasts, so the router interface for VLAN 40 is set with ip helper-address 10.1.8.5. It catches the broadcast and forwards it as a unicast to the DHCP server in another subnet, and the reply comes back the same way.
A DHCP relay (ip helper-address) turns the client's broadcast into a unicast to the server.

Exam tip Clients in one VLAN get APIPA but others are fine? Check the relay on that VLAN's gateway interface.

Objective 3.4

IPv6 addressing: SLAAC

SLAAC

Stateless address autoconfiguration (SLAAC) lets an IPv6 host build its own address. The router sends a Router Advertisement (RA) with the /64 prefix; the host adds its own 64-bit interface ID (EUI-64 from its MAC, or a random value for privacy) and checks for duplicates with Duplicate Address Detection. No DHCP server needed.

Exam tip SLAAC = no server tracks leases. Stateful DHCPv6 = server assigns and tracks addresses. Stateless DHCPv6 = SLAAC address + DHCPv6 options.

EUI-64 in one picture

EUI-64 splits the 48-bit MAC, inserts FFFE in the middle, and flips the 7th bit of the first byte.

MAC        00-1A-2B-3C-4D-5E
split      001A2B | 3C4D5E
insert     001A2B FFFE 3C4D5E
flip bit7  021A:2BFF:FE3C:4D5E
prefix     2001:db8:20:40::/64
address    2001:db8:20:40:21a:2bff:fe3c:4d5e
Building an IPv6 interface ID with EUI-64 from the MAC address 00-1A-2B-3C-4D-5E. Step 1: split the 48-bit MAC into two halves, 001A2B and 3C4D5E. Step 2: insert FFFE in the middle. Step 3: flip the seventh bit of the first byte, so 00 becomes 02. The result is the 64-bit interface ID 021A:2BFF:FE3C:4D5E, which follows the /64 prefix.
EUI-64: split the MAC, insert FFFE, flip the 7th bit.

Check yourself 1The label printer must always get 10.30.0.25 from DHCP, and no other device may get that address. What should Dev configure?

  1. A longer lease time
  2. An IP helper on the VLAN 30 gateway interface
  3. An exclusion for 10.30.0.25
  4. A DHCP reservation for the printer's MAC
Show the answer

Answer: D. A DHCP reservation for the printer's MAC

A reservation maps the printer's MAC to 10.30.0.25, so DHCP always gives it that address and nobody else.

Practice questions

Answer in your head, then open the answer. Ranger's Packet Trail has 483 questions like these for Network+, with mock exams.

Question 1A printer must always receive 10.1.5.50 from DHCP so users can find it, but the admin wants it to keep using DHCP. What should be configured?

  1. A shorter lease time
  2. A DHCP exclusion for 10.1.5.50
  3. A reservation for the printer's MAC
  4. A DHCP relay agent on the printer VLAN
Show the answer

Answer: C. A reservation for the printer's MAC

A reservation ties an IP address to a client's MAC (or client ID), so DHCP always hands it the same address.

Question 2Routers and servers on 10.1.5.1-10.1.5.20 use static IPs inside the DHCP scope 10.1.5.0/24. What prevents DHCP from handing out those addresses?

  1. DHCP option 3
  2. An exclusion range for .1-.20
  3. A longer lease time
  4. A reservation for each device
Show the answer

Answer: B. An exclusion range for .1-.20

Exclusions remove addresses from the scope's pool so the server never leases them. Reservations are for DHCP clients that should always get the same address.

Question 3Which DHCP option provides clients with their default gateway?

  1. Option 51 (Lease time)
  2. Option 6 (DNS servers)
  3. Option 15 (Domain name)
  4. Option 3 (Router)
Show the answer

Answer: D. Option 3 (Router)

DHCP option 3 is Router (default gateway). Option 6 is DNS servers, 15 is domain name and 51 is lease time.

Question 4A DHCP server at 10.0.0.10 serves several VLANs. Clients on VLAN 30 (10.30.0.0/24) get 169.254.x.x addresses because broadcasts don't cross the router. What should be configured on the VLAN 30 router interface?

  1. DHCP snooping trust
  2. A DHCP reservation for VLAN 30
  3. A static route to 10.0.0.10
  4. ip helper-address 10.0.0.10
Show the answer

Answer: D. ip helper-address 10.0.0.10

A DHCP relay agent (ip helper-address on Cisco) turns client broadcasts into unicasts sent to the DHCP server in another subnet.

Question 5A coffee shop's DHCP pool runs out by noon because customers come and go but keep their 8-day leases. What is the BEST fix?

  1. Turn on a second DHCP relay
  2. Create more exclusions
  3. Shorten the lease time to an hour or two
  4. Add DHCP reservations for customers
Show the answer

Answer: C. Shorten the lease time to an hour or two

Short leases return addresses from departed clients to the pool quickly, which suits busy guest networks.

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