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Difficulty: HardIPv6 Addressing and Prefixes

A network engineer is configuring an interface on a Cisco router for VLAN 270 using the assigned IPv6 block 2001:db8:abcd::/482001:\text{db8}:\text{abcd}::/48. Per organization policy, the 16-bit subnet ID field in the /64/64 prefix must be the exact hexadecimal equivalent of the decimal VLAN ID (27010270_{10}). The router interface has a hardware MAC address of `3C4A.9211.2233` and is configured with the command `ipv6 address 2001:db8:abcd:10e::/64 eui-64`. Which IPv6 global unicast address will be dynamically generated on this interface?

  1. 2001:db8:abcd:10e:3e4a:92ff:fe11:2233Answer
  2. B
    2001:db8:abcd:10e:3c4a:92ff:fe11:2233
  3. C
    2001:db8:abcd:270:3e4a:92ff:fe11:2233
  4. D
    2001:db8:abcd:270:3c4a:92ff:fe11:2233

Answer

2001:db8:abcd:10e:3e4a:92ff:fe11:2233
The correct option correctly converts decimal VLAN 270 into hexadecimal 010E (`10e`), inserts `FFFE` into the MAC address `3C4A.9211.2233`, and flips the 7th bit of the first byte (`3C` = `0011 1100` -> `0011 1110` = `3E`), producing the full IPv6 address `2001:db8:abcd:10e:3e4a:92ff:fe11:2233`.

Step-by-Step Solution

1
Calculate the 16-bit Subnet ID from the decimal VLAN ID
Decimal 27010=(1×162)+(0×161)+(14×160)=0x010E270_{10} = (1 \times 16^2) + (0 \times 16^1) + (14 \times 16^0) = 0x010E, which compresses to `10e` in the 4th hextet.
IPv6 hextets are written in hexadecimal format. Converting decimal 270 yields 010E.
2
Insert FFFE into the middle of the 48-bit MAC address
MAC `3C4A.9211.2233` becomes `3C4A:92FF:FE11:2233`.
EUI-64 generation splits the 6-byte MAC address into two 3-byte halves and inserts `FF-FE` (16 bits) in between.
3
Invert the 7th bit (Universal/Local bit) of the first MAC octet
First octet `3C` in binary is `0011 1100`. Inverting the 7th bit (bit 1 from the right in the first byte) changes it to `0011 1110`, which equals `3E` in hexadecimal.
IEEE EUI-64 standard dictates that the 7th bit of the interface ID must be inverted (0 for global scope, 1 for local scope).
4
Combine the /64 prefix with the EUI-64 interface identifier
`2001:db8:abcd:10e:3e4a:92ff:fe11:2233`.
Appending the 64-bit interface ID `3e4a:92ff:fe11:2233` to the /64 prefix `2001:db8:abcd:10e::/64` yields the final global unicast IPv6 address.

Key Concept

EUI-64 Interface Identifier Generation & IPv6 Hexadecimal Subnetting
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Topics

IPv6 Addressing and Prefixes

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