This is what happened to me: (P.S, I play on an iMac)
18:23:46 [sEVERE] java.net.UnknownHostException: 5.141.61.186:25564
18:23:46 [sEVERE] at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
18:23:46 [sEVERE] at java.net.InetAddress$1.lookupAllHostAddr(InetAddress.java:849)
18:23:46 [sEVERE] at java.net.InetAddress.getAddressFromNameService(InetAddress.java:1202)
18:23:46 [sEVERE] at java.net.InetAddress.getAllByName0(InetAddress.java:1153)
18:23:46 [sEVERE] at java.net.InetAddress.getAllByName(InetAddress.java:1083)
18:23:46 [sEVERE] at java.net.InetAddress.getAllByName(InetAddress.java:1019)
18:23:46 [sEVERE] at java.net.InetAddress.getByName(InetAddress.java:969)
18:23:46 [sEVERE] at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:137)
18:23:46 [sEVERE] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:426)
18:23:46 [sEVERE] at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
18:23:46 [sEVERE] Unexpected exception
java.net.UnknownHostException: 5.141.61.186:25564
at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
at java.net.InetAddress$1.lookupAllHostAddr(InetAddress.java:849)
at java.net.InetAddress.getAddressFromNameService(InetAddress.java:1202)
at java.net.InetAddress.getAllByName0(InetAddress.java:1153)
at java.net.InetAddress.getAllByName(InetAddress.java:1083)
at java.net.InetAddress.getAllByName(InetAddress.java:1019)
at java.net.InetAddress.getByName(InetAddress.java:969)
at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:137)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:426)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)