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# C:\WINDOWS\SYSTEM32\DRIVERS\ETC\HOSTS
127.0.0.1       localhost
172.24.13.49    Test	# 隣の席のやつのPCのIPアドレスにTestって名前をつけてみた

これでPINGを打ってみる

C:\>ping test

Pinging Test [172.24.13.49] with 32 bytes of data:

Reply from 172.24.13.49: bytes=32 time<1ms TTL=128
Reply from 172.24.13.49: bytes=32 time<1ms TTL=128
Reply from 172.24.13.49: bytes=32 time<1ms TTL=128
Reply from 172.24.13.49: bytes=32 time<1ms TTL=128

Ping statistics for 172.24.13.49:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

ふむふむ、HOSTSが有効なのは確認できた。

LANケーブルを抜いて、もう一度

C:\>ping test

Pinging Test [172.24.13.49] with 32 bytes of data:

Hardware error.
Hardware error.
Hardware error.
Hardware error.

Ping statistics for 172.24.13.49:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

エラーになった。そりゃそうだ。そしてふたたびHOSTS修正

# C:\WINDOWS\SYSTEM32\DRIVERS\ETC\HOSTS
127.0.0.1       localhost
#172.24.13.49    Test	# コメントしてみた
127.0.0.1       Test	# localhostならLANケーブル抜けててもOKのはず

C:\>ping test

Pinging Test [127.0.0.1] with 32 bytes of data:

Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128

Ping statistics for 127.0.0.1:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

うん、反映されましたね。そこで再びHOSTSを編集して、LANケーブルを挿してみる

# C:\WINDOWS\SYSTEM32\DRIVERS\ETC\HOSTS
127.0.0.1       localhost
#172.24.13.49    Test	# コメントはずす
#127.0.0.1       Test	# 今度はこっちをコメント

C:\>ping test

Pinging Test [172.24.13.49] with 32 bytes of data:

Reply from 172.24.13.49: bytes=32 time<1ms TTL=128
Reply from 172.24.13.49: bytes=32 time<1ms TTL=128
Reply from 172.24.13.49: bytes=32 time<1ms TTL=128
Reply from 172.24.13.49: bytes=32 time<1ms TTL=128

Ping statistics for 172.24.13.49:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

環境はWin XP SP1。注目してほしいのはIPアドレス。どうやら都度名前を解決しようとするみたい。でも、アプリケーションによるかもね。